What about times when someone is talking about the canon of Depression-era photographs or using i as a lower-case Roman numeral? Also, it's probably not possible to implement this feature completely because the list of proper nouns and other words that should be capitalized is very long and always changing. Furthermore, incorrect capitalization is just one of many errors that we see. There's also poor formatting, capitalization of words that shouldn't be capitalized, spelling errors, poor grammar, generally unclear writing, and so on.
I sympathize with your frustration, but it'd be far more annoying to have SE insist on inappropriate capitalization just because I happen to use a word on some arbitrary list of words for which capitalization is required. Some sort of automatic grammar checker that suggests but doesn't insist upon improvements would be great, and could address more than just the capitalization issue, but that seems like a taller order than the feature you requested.
I'd suggest simply downvoting questions and answers that are sloppy, and perhaps adding a comment to explain the downvote and encourage more attention to detail.