Part is the lack of a party system and part is the way votes have been made fungible. The politicians say we vote publicly so the public can see how we voted. This is false, they vote publicly so other politicians and lobbyists can see them do their paid duty. Literal bribery masked as lobbying. The lobby contributions are suppose to be public, and we see those. What we do not see are the paper bags of cash and the offshore wire transfers of huge amounts. The chicken feed we see under the rubric of allowed campaign contributions is a public sop.
Make all vote secret. You have a secret ballot? Why do the politicians have a pubic vote? = keep the gravy train in the USA flowing. In Canada we have strict party politics and you MUST vote as your party directs or get the boot.
So there you have it, the rich have 89% + of the politicians in their waist coat pockets - that is all they need.