Because their voters want them to. People love to act like 40% of Americans wouldn't vote for Trump right now if asked to, but they would. He could rape a baby live on TV and the evangelicals would tell their congregations that at least he's not a Democrat. The system isn't broken, it's giving the American population what they want. They just want stupid shit and more dead brown people.
Everyone's acting like Trump is a fluke but he's not. Republican voters would never primary their candidates to be less MAGA nor will they ever vote D. The best we can hope for is them going silent and not voting.
Yes, I'm aware that "did not vote" wins every US election. But that doesn't change the fact that the plurality of US voters chose Trump, and enough of them still do that it doesn't matter. The system is reflecting the will of the people accurately... if Republicans wanted, they could primary every single MAGA shithead and elect Republicans willing to impeach him. They won't. Because they still believe.