Where does capitalism mandate corruption? Yes, it is not realistic to assume there is no corruption, but capitalism in and by itself does not mandate corruption.

Obviously this all falls apart when capitalism can buy legislation. We are seeing how the USA is currently eroded by a few oligarchs.

They're not saying it 'mandates' it as law, but that the systematic incentives inevitably lead to corruption. The ability to buy government is irrelevant - this is just the easiest method right now of converting money into power. If there was no government to buy, private business would execute that conversion themselves by ruling over people and enforcing their wishes directly.

Anything that involves humans will have corruption.

Society needs somethings to try to stop corruption wehther government rules or non government actions.

Under pure capitalism what stops this?