There are detectors for VMs, and modifications to allow VMs to evade those detectors. It's an arms race.

Example: There is (was? I don't actively follow the community) a patch set for a particular piece of VM software that made it undetectable to anti-cheat in games.

While I don't use said software (I have a casual interest in it only...would be nice to get more games working on Linux), I have to disclose that I'm against anti-cheat mechanisms. I'm a software engineer, and I've worked on a few smaller games, and know the overall structure of bigger ones, and I don't think I've ever seen a game use good practices in multiplayer. Instead, they usually rely on client side code and lean on anti-cheat software to stop cheaters.