more eyes; if a game contained some kind of malware, there will likely be reports made on the game discussion page (hosted on steam), and game will likely be taken down with enough complaints

i've seen this happen before when some random game added a cryptominer in a silent update

It's even worse than that actually. Steam seems to have no problem with apps or games that require elevated/admin/root permissions to run, while at the same time a lack of code signing on those executables, even if the permissions on the file/folder allow modification by any user on the operating system. Some games carry recommendations by the developers, support, or the communities themselves to open up those permissions or run the game as an administrator, to "fix the problem". If you dare raise a concern, you'll be heckled and mocked.

The carelessness around security is pervasive in the gaming community and Steam isn't helping the situation, likely because it would be bad for revenue.