> I don’t really understand why so much of science is funded by the government instead of by private philanthropy.

Two reasons.

First, private philanthropy is neither sustainable nor sufficient in scope.

Second, because government funded science is free from having to produce immediate results in order to satisfy the whims of a specific patron such that funding is continued.

> It seems like an obvious channel for corruption of science.

Yes, relying on private entities to fund scientific research does seem like an obvious channel for corruption.