Anyone with half a brain could spot a karma farming account. When I first became a mod of a very large sub there I looked at the recent ban log and spotted some accounts banned a couple months back for karma farming. They were being used for things like promoting drug sites, crypto, etc.
The algorithm used karma and history to help filter/restrict accounts. The problem was that not everyone was committed to doing that. Most of the meme subs had mods who just didn't give a shit and when you sent them a modmail with: 'Hey these are clearly bot accounts reposting word for word popular posts including the links (which is a really good indication it's a script', they just wouldn't do anything or just very aggressively respond that they wouldn't do anything. Henceforth their subs became ground zero for botters and scammers who wanted to build history on an account.
Reddit had an automated process, as far as I could tell, for banning/restricting these kinds of account. If enough large subs banned them in a period of time, it would seem like their accounts would be suspended almost immediately. So if they happened to post in the wrong group of subs when I spotted them, I'd modmail all those subs and they'd all ban them and the account would disappear, but if they hit the right subs where the mods didn't care, then they wouldn't, and it would take a lot more work to get them dealt with.
Karma is a good way to track participation, but if people ignore/abuse the system it falls apart.
The fact that reddit didn't have a process in place to track accounts posting word for word (including link) reposts and immediately ban them was really weak. Also the fact that the algorithm checked karma and history but the admin gave their blessing to subs like freekarma4u because some subs had karma requirements was a bit of a joke.