To add to what other people said: As a casual user who didn’t go deliberately looking for bots, I mostly caught them when they posted a comment that was a complete non sequitur to the comment they replied to, like they were posted in the wrong thread. Which, well, is because they were–they were copied from elsewhere in the comment section and randomly posted as a reply to a more prominent thread. Ctrl+F came in very handy there. (They do sometimes reword things, but generally only a couple of words, so searching for bits and pieces of their comment still usually turns up results.)
Also, the bot comments I caught were usually just a line or two, not entire paragraphs, even if they were copied from a longer comment.
Normal people wouldn’t. Unfortunately, there are a lot of assholes, stalkers, and people who are salty they got downvoted and want revenge.
Ever seen people on Reddit say “Whoever downvoted this, go fuck yourselves?” I can guarantee that, if they knew who downvoted them, they wouldn’t keep their reaction contained to an edited comment.