Well you would think that, but if that was the case people wouldn't lose their accounts in the first place, but I've lost count of how many times I've seen people complaining that their blockchain.info account has been hacked and they've lost their funds, and the conversations always go like this:
Q Did you have 2-factor set up?
A No
Its mainly newbs who lose their funds but it's those newbs that likely wont set up 2-factor either.
Ok, I would say that you are quite right. But I think 2FA would anyway help to REDUCE the hacks and maybe it would discourage potential hackers from trying to get accounts. All in all it's a cat-and-mouse game between the security and the hackers, they will ever find vulnerabilities or a hole to reach their targets.