Because people are dicks. It's not about the challenge for cheaters, at least for most of them. The bulk of them do it to ruin the games of others. The minority are the ones who cheat and try to play it off like it's skill. For them, it likely derives from being bad at the game and wanting to be good, but the vast majority of cheaters are just assholes who want to make sure no one has a good time.
And cheating is always worse in free to play games because there is no consequence for getting caught. You can just create another account and you're right back to cheating. As someone who plays Team Fortress 2, it had few major cheating issues when it was a twenty dollar game. Once it went F2P, the cheating became a lot more of a problem and it still is. Fortnite Battle Royale, not coincidentally, is also free to play.
It's not nearly as much of an issue in paid games because, obviously, you'd have to buy another copy of the game every time you get banned, at least for digital purposes. Even a five dollar game isn't really worth it to cheat in, because you're out five dollars every time you get busted and that adds up over time. I'd also imagine that the bulk of cheaters aren't wealthy, successful people. They're likely mostly asshole kids.
