At the size of Reddit, it's impossible as a mod to keep track of every account's individual activities. A mod of a meme sub with millions of subscribers isn't going to vet every user in the sub. To recognize a karma farming bot, you would have to know that their content is reposted. But if you're viewing the bot's post for the first time, how would you know? You would just assume that the post is original and upvote it. The karma bots also crop or filter their reposted images to make sure "repost identifier" bots don't catch them.