Edits should make a post better. But what does "better" mean? It isn't a simple matter.
At one extreme, take a canonical post that gets heavy, ongoing traffic and contributes a really good question or answer. A single typo may be glaring and detract from every use. Polishing such a post with a minor fix can contribute value to the site.
At the other extreme, take a garbage post that not only contributes nothing of value, but also contains extensive errors of spelling, grammar, etc. Maybe it's a rant, or a completely misguided answer. Fixing all of the English mistakes still won't make it a useful post, so it isn't really an improvement.
Take another example, say a trivial question from 7 years ago that was of interest to only a few people and received an answer. But the matter is no longer relevant, and the thread has been buried and inactive since the day it was written. Someone does a search on a bad grammar pattern or common typo, looking for posts to polish, and stumbles across that thread. They could polish the heck out of it, turning it into pristine English, but the thread is no longer of use to anyone. Is it worth the time of three reviewers to review the edits, or worth bumping it to the main page where it will take oxygen from current questions?
One more example--a question that is closed or on-hold. Any edit automatically puts it in the reopen queue where five people will need to spend time reviewing the question to see whether it merits reopening. If an edit fixes things like spelling and grammar, but doesn't address the closure issue, a lot of people will waste time reviewing a question that will have better English when it is deleted. That isn't really an improvement.
"Better" should be from a site perspective, not just the text in the abstract.
Also, we get questions where the author has obviously not invested any effort before coming here asking other people to invest their time responding. Those questions aren't well received. If someone is going to do edits that three other people will have to spend time reviewing, the same logic should apply. They should put in the effort to fix as much as they can of what is needed. The push-back is generally when extensive fixes are obviously needed, but the editor just does a few items of low-hanging fruit.