I think a wrench and 20 minutes of time is (1) a higher barrier than you think (not to say it justifies the lack of bidet) and (2) is an underestimate of how inconvenient installing it is. I installed a bidet a few years ago and it probably took 2+ hours and ended up leaking. When I moved, I opted not to install one again and instead follow #2 with a shower. 

However, I agree bidets are great modulo the installation, and I will try to buy a toilet with one pre-installed.

As a Bay Area native, I have seen bidets retrofitted onto toilets at probably 3-4 places, all of which were at rationalists' houses.  So if anything, it seems to me that rationalists are more bidet-friendly than the average American.

I don't think I've ever heard Americans talk about bidets—except one friend saying that his grandmother had visited Japan and used their toilets and reported "I felt like I was on the starship Enterprise!".

In what I think is the same reference class, I don't think I've ever heard someone mention using wet wipes in the bathroom.  I have noticed seeing them in one or two people's bathrooms (other than my own); and on one occasion I idly asked someone what was in a package he received and he said "wipes" (and possibly seemed reluctant to talk about it, and I didn't ask further).

I think people just don't talk about that kind of bathroom stuff, at least in America.  Chalk it up to puritanical prudishness or something.  Which does make it hard for innovations to spread by word of mouth.  Unfortunately.

Personally I don't like the feeling of having a wet asshole

Just a single data point: a housemate of mine installed a bidet in one of our toilets, and as a result of the way the bidet changed the overall shape of the toilet and the positioning of the seat and so forth, my genitalia were physically smushed up against the front of the bowl when I sat. Like actually touching the porcelain. This was unpleasant.

I don't consider this a dealbreaker for bidets =P. But it's a bit of weight on your second point; there are ways bidets can fail to be great and fail to be a harmless improvement over the status quo.

I'm a bit hungry for ... the thing that bidets are a metaphor for, here? Like, I think there's an interesting object-level question about the adoption of (almost strictly) superior ways-of-being, but I'm more curious about something like "what does the non-adoption of bidets tell us about how we are probably thinking and behaving sub-optimally elsewhere?"

Last summer I went on a week long backpacking trip where we had to carry out all our used toilet paper.  

This year, I got this bidet for Christmas:  https://www.garagegrowngear.com/products/portable-bidet-by-culoclean

You could carry one with you so you are no longer reliant on having them provided for you.

Bidet usage may be the strongest example of conformity bias ever. Go in Italy, and literally everyone uses a bidet (there is even a law that makes it mandatory). Cross the border with France, and suddenly there are no bidets anymore.

I can't even start to tell how incredibly unlikely would be to meet an Italian claiming not to use it. Italians face a lot of conformity pressure for other things, like being Catholic, drinking small amounts of wine or paying their taxes, and still it's not so difficult to find Italian people claiming to be atheists, non-drinkers or even tax evaders. But I've never ever met anyone in Italy claiming not to use bidets. This is one of the few cases where I am quite certain that the small minority got it right, as conformity bias alone cannot be that strong, and the only downside of a bidet is that you have to reserve some space for it in the bathroom. If you are a bidet user, everyone else start looking like dirty savages, and every Italian lacking a bidet abroad will surely get mad about it.

There are a near infinite number of ways in which I could probably make minor improvements to my life. But it's not even worth examining them, let alone trying them, if the cost of examining and trying them will, on the average, be more than the benefit. Note that it doesn't matter that any particular example will be worth it--I don't know this in advance--so telling me "bidets are really great" won't affect this position.

Furthermore, I'm more skeptical than usual about any example that involves small actions that a human has to do routinely. That's a recipe for creating cognitive biases; if the thing really isn't very good, people will rationalize it away as being good so as not to admit to themselves that they're wasting so much time and effort. And doing many small actions related to necessary human bodily functions create positive reinforcement by their association with those necessary functions.

I'm also more skeptical than usual of things that go "make this definitely existing expenditure for this possibly existing benefit".

Also, Chesterton's fence.

I've wanted to install a bidet for 8+ years. However, I've always had higher-priority projects.

Costs that deter me:

• What for you is a 20-minute project will be 4-8 hours for me because it involves plumbing (and I want it to not leak). The fastest plumbing project I've ever had (cleaning the p-trap beneath the bathroom sink) took 1.5 hours.
• Hiring a contractor will be $100 because I live in a high-rent area, and they need to cover the expense of coming out. It will take me 1 hour to choose, schedule, and oversee a contractor.
• I don't know how to choose a bidet. It'll take me 2-4 hours to research them.

The benefits are lower for me than for you:

• I estimate it will save six rolls of toilet paper per year. That comes to about $20. If I value my hours at $50, hiring a contractor is $150, choosing a bidet is $100, and the bidet itself is at least $35. The sum is $285, a 14-year pay-off time.
• I mainly want the bidet for comfort and because it will make me cleaner. Comfort and hygiene are lower-priority items for me. $20/year of extra comfort drops the pay-off time to 7 years.

BTW: Aella, a rationalist-adjacent Twitter user, mentioned that she uses a bidet.

I have never used a bidet because I don’t like the idea of a spigot that has sprayed a bunch of other people’s butts also spraying my butt.

I’m open to arguments that I’m making a bad life choice here.

My Bay Area rationalist house has a bidet.

For me, they sound great in theory but I worry about them making an absolute mess by shooting water everywhere if I don't use it exactly right (I have a hard time imagining it being effective without sufficient pressure to be potentially messy). I also don't expect cold water on the undercarriage to be pleasant. Based on what I've heard, I'd guess it isn't hard at all to learn to use one and get used to the cold blast but, not having used one personally, I'm not confident that the payoff is worth the hassle of researching, purchasing, and installing one (which would require far more time than 20 minutes--I guess 2 hours absolute minimum if nothing goes wrong).

I'm sure my skepticism isn't especially rational, but it may be useful as a representative perspective of those who haven't tried one or thought about it too much. I have actually considered getting one before but my laziness and lack of plumbing skills are a significant barrier even if my trepidations were assuaged.

There is now a South Park episode titled "Japanese Toilet", which depicts them as being insanely good (obviously exaggerated) (I haven't seen more than the clip).  I expect this will cause some fraction of viewers to become curious and look into the reality, and some fraction of those to try out something with a bidet.

I got finally convinced to get one for my house after enjoying one at a friend's house (rationally adjacent group house). Installation was a bit of a pain, because it turned out that on the toilet I had it caused my genitals to be smooshed against the front porcelain. I fixed this by buying the same brand of toilet as the bidet I bought. They were engineered to work well together. Worth, but not a small hurdle to overcome.

I think a better question is "why don't Americans use a bidet?" even though they are standard in many parts of the world. My guess is that the perceived benefit is not considered worth changing the habit, especially given the perceived effort required and perceived unknowns and potential downsides. There is nothing especially "rationalist" about it. Note the emphasis on "perceived".