That's because politics in the US requires money and no politician can rise to the mainstream without money. Not only that, if the capital interests saw you rising in local/state elections, then they could fund your opponents and whittle you down.
We are a nation ruled by money/capital. NK is a nation ruled by the kim family. The chinese are a nation ruled by the communist power. Someone in the US going against corporate interests has as promising a political future as a north korean politician railing against the kim family or a chinese politician going against the communist party.
Everyone, including the politicians, have to play by the rules of the system.
Look at the past election, both trump and clinton were pro-corporate/capital politicians. Hillary was probably even more pro-corporations/neo-liberalism than trump. But democrats will argue that hillary cared about the lowly workers.
But can you really blame the clintons, trumps, obamas, bushes or any other politician in the US? They are just playing the game within the system.
Ultimately, in the US, both the republican and democratic parties serve one master - the capital owners. The US is really a 1 party nation if you look beyond the silly social issues that the republican and democratic play off of each other to distract the masses.