To my knowledge the problem isn’t getting together the funds for any single “contribution”, it’s having the funds to pay the politician off over and over again. Sure, half a million dollars sounds like a lot more than a hundred thousand, but how many times can that half million be successfully crowdfunded? It’s much more reliable for the politicians to just accept the smaller but more consistent “contributions” from the more wealthy parties.

On top of that, outright bribery is illegal, attempting something like that is liable to get you arrested.