1. It's capricious.
2. It doesn't catch actual bugs, and then you have to wait to fix them.
3. Apple doesn't engage you easily on questions of whether something or another will result in rejection.
4. Apple changes the rules and block random releases, without communicating rules changes until they reject you.
5. Apple makes it harder to plan marketing campaigns.
I haven't had a new app rejected in probably over a year, and only rarely have they blocked one of my releases, but every time it sucks, and I don't know why they bother.