What is the point of College? Specifically is it worth investing time to gain knowledge?

First, the standard answer: Bryan Caplan's The Case Against Education. Short version: education is about signalling to future employers how smart/diligent/willing-to-jump-through-hoops/etc you are. Skill acquisition is mostly irrelevant. This is basically true for most people most of the time.

That said... I personally have gotten a lot of value out of things I learned in courses. This is not something that happens by default; the vast majority of my classmates did not get nearly as much value out of courses as I did. I'll list a few things I did differently which may help.

Avoid nontechnical classes: this one is kind of a "well duh" thing, but there are some subtleties. "Technical" should be interpreted in a broad sense - things like e.g. law or languages aren't technical in the sense of STEM, but they're technical in the sense that the things they teach are intended to be directly useful. By contrast, subjects which are primarily about aesthetics or history or critical theory are not really intended to be directly useful.

Decreasing marginal returns: the first course in any particular field/subfield is far more valuable than the second course, the second course is more valuable than the third, etc. This suggests going for breadth over depth. In particular, I recommend taking one or two courses in many different fields so that you can talk to specialists in those fields without being completely lost. You don't need to become an expert yourself; much of the value is in being able to quickly and easily work with specialists in many different fields. You can translate jargon and act as a human interface [LW · GW], and you can easily jump into many different areas.

General-purpose tools: focus on fields which provide tools applicable to many different domains. Most of applied math qualifies, as well as computer science, economics, and law. Ideally, you take one or two courses in some general-purpose subject, then run into applications of that subject while sampling other fields. By seeing it come up in different contexts, you're more likely to remember and use it.

Summary of this point and previous: go for depth in general purpose tools, and practice those tools while gaining breadth in other areas.

Use available resources: I've covered about as much material in open courseware as I have in in-person classes. I've watched online lectures, I've read textbooks, and I've audited courses. (I've even audited classes at universities where I'm not registered - professors are usually happy with people just showing up.) In college, I'd often watch a semester's worth of online lectures on a subject before taking a class on the subject; material is a lot easier to follow when you already have a general idea of where things are headed and how it all slots together..

Have a stock of problems: as you learn new tools, it's useful to have a handful of problems to try them out on. Hard open algorithmic problems like integer factorization or P vs NP or graph isomorphism are great for testing out all sorts of applied math/CS tricks. "How could I use this to start a company and make a gazillion dollars?" is a problem which applies to practically anything. The problems should be things you're interested in and enjoy thinking about, so that you'll find it worthwhile to try out new tools on them even though most of the tools don't actually yield breakthroughs on most of the problems.

This point and the previous one help a lot with actually remembering things and being able to apply them in-the-wild.

Optimize: at my college (Harvey Mudd), it was very easy to tell who had actually tried to optimize their course choices - it was the people who used the "build your own major" option. We only had half a dozen majors, and the course requirements always included things which weren't really what any particular person was interested in. If you wanted to cram in more courses you were actually interested in, while avoiding irrelevant courses, a build-your-own major was the way to go.

More generally, you'll get more out of classes if you read through the whole course catalog, mark the classes which sound interesting, and then optimize your schedule to focus on those classes. Sounds obvious, yet most people don't do it.

Be comprehensive: you're not going to know everything, but you can learn enough that nothing is very far from the things you do know. You can learn enough that you at least have some idea of which things you don't know. You can learn enough that, even if you don't know something, you've probably heard of it, and you have some idea of where to learn about it if you need to. The key is to aim for comprehensive knowledge of the world - you don't need to know every little detail, but at least get the broad strokes of the big things. Anytime you don't have a clue about something, pay attention to it, and look for the most general subject which would give you some background about that thing.

Math, physics and economics are particularly useful for comprehensive foundations - they give you the tools to solve practically anything in principle, though you'll often need more specialized tools in practice.

A few thoughts, none of which are actually answers:

a) Knowledge and forgetting is tricky. There's a very good chance that even most of what you've forgotten is still there, and you'll learn it again much more quickly should you ever need it at a later time. Your class didn't give you knowledge, but it gave you the way to (re)acquire that knowledge when you need it.

b) Both the value of knowledge and the ability to use it is highly variable. the way it worked for me could be completely different than the way it will work for you. College is great for some subset, suboptimal but still positive for others, and negative for still others.

c) (perhaps most important) there's a pretty strong argument that knowledge isn't the primary value of college. The people you meet and/or the credentials you gain could well be the more important things, with (temporary) knowledge as a gateway to both - good (temporary) learning leads to hanging with smarter people and with better grades/degrees.

d) when asking "is it worth it", you must always include "compared to what"? What's your next best activity? Can you get a good engineering-related job without a degree? If so, maybe that IS better for you.

I agree with Dagon and johnswentworth's answers, and I would like to particularly emphasize one point. For me, as a mechanical engineer, most of the time I do not use most of the things I learned in school (and indeed I have forgotten most of them). Learning to do my job took months of studying field-specific stuff that most engineering schools don't teach. But if I had tried to learn it without first taking a lot of engineering courses, it would have taken years instead--and almost no company is willing to train someone for years. Think of education as turning yourself into a minimum viable product. It might be better in principle to intersperse work and school--either work as an engineer and then take time off to study, or do some study on the side. But you can't do either of those until you're skilled enough for someone to want to pay you.

Also, every now and then, some piece of knowledge from a course turns out to be useful, but you can't know in advance what it will be. If you haven't already studied it, then when the situation arises where you could use it you won't be able to tell; and you can't look something up unless you already know it's important.