I'm sure there are game devs that try to install rootkits for no good reason, but preventing cheating in multiplayer competitions can unfortunately be a legitimate reason to ask "Hey $customer, I need to take full control of your machine, and it can't be a VM".

You basically have three options:

- Cross your fingers and pray that the game company isn't going to do anything malicious to your machine

- Keep your gaming PC completely isolated from your private data

- Accept that you won't be able to play some competitive games, or the competitive modes of some games (eg. the default CounterStrike multiplayer runs with usermode anticheat, but competitive matchmakers like FaceIt require you to install a kernel-level anticheat)