Because the crooked politicians usually bring something back to their constituents. And people vote on their local politician.

So the US Senate is usually filled with moderate voices (since they have to appeal to millions of voters) while the House of Representatives is filled with crazies who were elected by an order of magnitude less than senators.

Then you have the incumbency effect which makes it pretty hard to lose your next election so long as you have name recognition and a gerrymandered district, unless something really crazy happens with you (sex scandal, fake-news, corruption indictments, going to prison, etc), and even then sometimes the effect is still strong enough to elect you to office before you have to be removed and have the State Governor appoint an UNELECTED replacement who will then have the incumbency effect bestowed upon themselves.

All in all, the US makes it hard to elect people who will "progress" and not just "maintain".

And don't get me started about off-year elections, special elections, primaries, voter ID laws and various other legalized voter suppression (felons, poor people, minorities), and a plethora of other ways that we use to confuse citizens about voting.