I've done my bachelor's and masters in civil engineering at a state university. I did master's research and a thesis. I also taught classes as a TA (teaching assistant, the full lecture hall enrollment is divided up into smaller classes of ~20 to do labs/example problems/etc etc which meet 1-2 times per week depending on the class). Like you, I made up my mind about career choice before I even went to college (junior year of high school) and I haven't deviated from that path. But not everyone is so decisive. Many college students are still trying to find their calling 1,2, even 4 years in.
1) In this age of technology, there's very little that you can't teach yourself. Having a degree isn't much more than a piece of paper that ensures employers that you have a base requirement of skill set for the work you plan on doing. But you may find that that "pointless" piece of paper is extremely valuable.
2) Everyone in college (call it college for simplicity) has varying levels of knowledge in various topics. You seem to be pretty well educated in the things you've mentioned, and those classes may be trivially easy for you. But the person sitting next to you may be seeing this for the first time. Also, topics that you're most interested in may be absolutely boring to the person sitting next to you, or the 10-100 other people sitting in the classroom. You'll also likely find that you don't know everything about everything. Some topics may not interest you, or you may not see how/why/when you'd ever use that topic. But taking a class (whether required or elected) on something outside your knowledge base could introduce you to a new area of interest, or simply teach you an alternative skill set that can compliment your current repertoire.
3) If you're motivated, self study is great. However, it's also inherently specific because you're only studying things that [you know] interest you. You've already alluded to sticking to the few things you know because it's comfortable. That's great IF you can find a job/career using the very specific set of skills you've taught yourself. But it's oftentimes beneficial to have a more well-rounded knowledge to make you eligible for a wider range of jobs. Also, let college motivate you to dig deeper on your own into topics that are covered in classes. The unfortunate truth is that each class must cover a predetermined amount of subject matter in a finite amount of time. If you take a step back, you'll notice that most/all college courses only cover the various subject matter at a cursory level. Don't expect to learn everything you'll ever need to know in college, you'll be massively disappointed.
4) Be respectful to your professors and fellow classmates. Yes, YOU are going to college to benefit YOU, but so is everyone else. It's not fair to others if you're asking all the questions and driving the discussion. If there's something your'e interested in knowing more about, go to your professor's office hours and/or do some research on your own. Speaking from experience, professors are much more accommodating to questions during office hours than derailing interruptions during class. More-so if your questions are intended to give you a deeper understanding of the topics rather than most students that just go to office hours to get answers to homework questions. Hint-hint, bosses don't like it either.
It's great that you're thinking independently. Far too many college students just follow the "hive-mind" and mindlessly complete tasks to get a grade. Try to deeply understand how/why.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod