alt Hacker NewsPC games don't have to do this. Steam doesn't have a policy that achievements must prevent cheating. This is entirely a developer choice.
The main reason cheats have disappeared from mainstream games is because many of them prefer to sell them to you as micro-transactions, item pack DLCs, etc.
Steam has the weakest enforced terms of all the three that I mentioned, but it does have terms in its contract with developers for how Achievements are supposed to unlock.
But most "triple A" games don't just release under Steam's weaker terms, they also want Xbox and PlayStation certification, which have been strongly enforced.