Hindsight is 20/20. Everything is obvious, once someone already paves the way to the solution.

This is why, in principle, there is the "prior art" clause. It's just not enforced well enough in the software realm--but the mechanism to prevent patenting something widely known a posteriori does technically exist.

Well the non-obvious requirement of patents to be granted in the first place is defined such that experts in the field would not come up with it in response to the problem it solves. This is not the case with most software patents.