Congressmen have to get reelected, so over the years they've been glad to abdicate power to the executive, the judiciary, and the unelected bureaucracy. Anyone but themselves, so they didn't have to sign their names to the unpopular policies they wanted. They still got what the ruling class wanted, but indirectly, so it rarely threatened their incumbency. Whatever happened, they could tell the votes back home, "Sorry, we tried to pass/stop such-and-such, but we don't have any control over the president/courts/bureaucrats. Can't blame me."

It worked pretty well as long as the ruling class were all pretty much on the same page about most things, with some "social issues" differences between the parties that they used for campaigning but never quite acted on. It works less well if different factions start competing and going against the status quo for real.