Some time ago, ELL as a community decided that sometimes it's best to leave some minor mistakes in questions, because those mistakes might help other users more accurately gauge the experience level of the learner asking the question. So that's one reason we don't correct "everything" written in ELL.
Answers, though, are another matter – particularly accepted answers. In that case, I agree that minor errors ought to be corrected. As to why they aren't, I'm guessing that it's simply because those who spot the errors are not correcting them.
No Stack Exchange has a senior editor who reviews all answers, or all accepted answers. It's up to the community to fix these up as we find them, so it's no surprise that some go uncorrected.
As a footnote, I won't chastise anyone who leaves such errors alone. Everyone has their own reasons for participating. Some like to read answers, others like to leave answers. Some like to improve tags, some like to improve titles, some like to correct minor mistakes. Some like to leave flags and some like to leave things alone. It's all good. This diversity is what makes us a community and it's what makes the community work.
If a few folks suddenly felt like it was their duty and obligation to proofread and correct every accepted answer, they might well burn out, and we could end up losing some very valuable members of our community. Fix 'em as you find 'em, but if fixing them is not your thing, that's okay, too.