State governments have simpler and more reliable ways to attack the system. Like armies. Buying off the miners would require at least a few hundred million dollars—you'd need to buy the hardware to pull off an attack like that, not just rent it for a while. And you still wouldn't manage to get anywhere near the full market cap by shorting Bitcoin while you do it. You'd never find anyone crazy enough to loan you that much. A small fraction, perhaps. Not enough to cover your expenses.

And in the end, after your massive investment, the community would just make some trivial change to the protocol and completely ignore the attack. Really, if this sort of thing was so easy then everyone would be doing it. You're certainly welcome to try.