For the janitor, sure. And, yes, non-competes are beyond ridiculous for janitors, fast-food workers, and the like. For senior people? It's actually a pretty small industry. If an employer actually cares, they can find out pretty quickly where an ex-employee has landed especially if they're in a visible externally-facing role.

As you say, under most circumstances most companies don't actually care even if they have some boilerplate non-compete clause. But some do, especially in places where close client relationships are important or where it's a senior person moving to a direct competitor.

At the same time, someone who signs it might not know any of that. In that case, the contract does become "valid", because no one is going to challenge the validity of it.

You and I might know these clauses are unenforceable garbage, and we might be able to get a lawyer to send a nasty letter to the former company if they attempt to use it. However, that situation is not common for everyone, and unfortunately the most vulnerable in our society are the ones who get punished because of it.