The anti-cheats are there to detect the cheats, not necessarily stop them (though they usually do that too, at least the basic ones). When they're detected, they can ban accounts or whatever. They usually do this in waves to prevent cheat developers from knowing exactly when their cheats were detected and what triggered the anti-cheat.

Most games suffering from cheaters are at least partially free. LOL, Valorant, COD, CSGO, Fortnite, Overwatch, Various MMOs.

Getting a new bot-leveled account for any of these games costs somewhere from 0.30€ to 5€. Ban one account and they just use the next one for pretty much free.

Most of these games do account ban waves every few weeks to months, but if its this easy to create new accounts that's useless. IP bans aren't possible anymore since public IPs are dynamic and change every few days to weeks. Hardware bans, while technically possible, can still be circumvented easily by spoofing your mac-address and serial numbers.

The only way to minimise cheating would be linking your Social security number (or the equivalent for your Country) with your account, which leads to a lot of privacy issues. And even that isn't foolproof. LoL in Korea already uses this system and still has issues with a lot of trolls, scripters and wintraders.

Because having a low barrier of entry and smurfing is part of the business strategies of these sort of games.