My guess is that it's not necessarily that people at the patent office have a hard time telling if something is obvious, so much as that the incentive structures at play are all wrong.
I imagine that, if a patent reviewer denies a patent application, they potentially have a fight on their hands when their decision gets challenged. Whereas, if they approve it, any argument ensuing from that decision will be the court's problem. It's a classic "keep your head down and don't cause any trouble for yourself" situation.
Assuming I'm right on that guess, I'd further guess that it's also a classic "I'm not paid enough for this shit" situation. I don't have any friends at USPTO, but I have friends who work in other government offices, and, from what they've said, I would assume that that is a deciding factor in a large percentage of decisions that are being made by rank-and-file government employees across all branches. And I doubt patent clerks are any more likely than any other kind of clerk to be getting paid "dealing with this shit" money.