I guess it's different for each of us, but I would be surprised if you think EVERY course you took is useful. There must be one or two?
I used to read a lot of history and humanity books so I don't really want to spend money on any such courses. Plus there are so many free ones online so I can take whatever I want instead of being forced to take something I might dislike.
Some universities offer independent studies which is cool, in which people can take courses they want. But for OP's cause I think just building a bench by himself and working on open courses is better, unless that degree is really useful.
University degrees provide (among other things): Job training, a broad, well-rounded education, and a piece of paper at the end that certifies you got both. You may have independently studied the arts long ago, just like you may already have been a math or programming wizard, but the university doesn't know this and cannot certify it in writing.
Reasonable people can argue whether or not universities -should- just be single-subject job training. But as it stands today, they aren't.
I agree with you though: If OP just wants training in a single discipline, there are tons of online engineering courses, many of them free. That's not what a university degree is for.