jeez.
well technically you COULD teach yourself say, classical Chinese or quantum physics online--if you were lucky enough to get the resources.
however,
-you wouldn't learn it as quickly or effectively.
-there are some things (eg. second language conversation skills; writing skills in general, and a host of other things) that would be too hard or impossible to learn on your own.
-try getting into grad school with no post secondary academic record on the basis that you "taught yourself online."
-the professor knows more than you do, and if you go to a good school, chances are he/she knows more than wikipedia (or sometimes even the textbook) on the topic of concern.
-WOULD you actually do all the work and learn all the material you would have in college? the answer is no. you wouldn't write a 5-page essay for fun, let alone a 40-page essay. and if you're working full time as well, you simply wouldn't have the time to put into studying (or wouldn't want to). have you tried teaching yourself an academic subject? it's hard. I have tried teaching myself concepts, grammar, vocab. etc. in my major, as well as other languages (when I was taking time off to work) and I would consider myself fairly skilled in languages, but even when I used the university level textbook I still didn't get as much out of it as when I took the university level course, and also it took a lot longer.
If you still want to know specific things I have learned that I didn't or couldn't learn when I was teaching myself, ask away.
if I were you, I would re-assess the ideas you have about college. maybe it isn't for you, after all it isn't for everyone, but your post made it seem like you thought college is useless/a waste of time for everyone.