I think partly because I never had the fine tuned sense of social interaction that many are naturally blessed with and paired with a dangerous tendency to let pigheaded curiosity be a guiding principle.
But, the the stuff it has taught me is indispensable. It was never taught explicitly, but without school I would have grown up completely oblivious of the invisible walls, thorns and highways that inhabit the society we live in.
Like if you want to infiltrate into a rebel base as a spy, you would want to spend a lot of time learning their mannerisms and adopting a persona. School helped me tremendously in fitting in, because my type of person would have certainly ended up an outcast in this society, if I was to be homeschooled.
I read this essay a few years ago and it really resonated, it does obviously a way better job at putting my feeling into words: http://www.paulgraham.com/nerds.html
My small town school had instances of missing teeth, broken jaws, one kid beaten far enough to require hospitalization for 2 weeks, etc. 10 years later, the popular kids say that there was no bullying and it was all intended in good fun. My therapist didn't even believe me at first when I discussed the things that were going on at my school, but luckily I do have some reminders of those times on visible places.
Your school sounds awful, I'm sorry. I went to a pretty diverse school with >2000 students, so there was some violence, but it was either mutual or gang-related. Petty teenager dickishness existed, but nothing like the kind of Stephen King bullies you describe. As far as I know, at least.
I admit I could've been oblivious, but for that to be possible already indicates a friendlier environment than these descriptions.
Personally I learned nothing (edit: not much) in school, was bullied, it was an awful experience and all I remember is watching the clock all day waiting for it to pass while daydreaming. Elementary school was like one year to learn to make additions, one year multiplication, for how many hours in school? To finally get a lot of children struggling to read in 6th grade. Middle school was more or less the same for me. I feel like I was the subject of an ideology too.