[achompas]

This isn't a matter of corruption--it's an issue of public choice (or public economics, whichever you prefer).

What politician would scale back SS/Medicare when retirees are one of the largest voting blocks in America?

Governments also don't necessarily experience more corruption over time. Just look at overthrown dictatorships. If you said democratic governments, like the US, I'd agree with you, since the accumulation of wealth shifts power around.