Are you asking why it's not perfect? That's an impossible standard. In general app approval seems significantly ahead of where it was in past years.
"They have the resources to pay cheap reviewers who are probably better than this guy."
I'm going to go out on a limb and say that most large businesses have the resources to hire people better than their worst employees. Again if you're asking why their staff is not all 5 star employees who never make mistakes I don't even know where to start.
"Why not allow all apps into the store as long as they pass some security checks done by Apple (no downloading executables etc)?"
Because they don't want spam, porn, hate apps, whatever. It's easy to make sweeping judgements about what's best when you aren't actually managing a 200 billion dollar brand.