I think it's because for so long they've relied on Democrats/people on the left (I say that because it's not just in the US, it's become the right-wing tactic all over the place) being serious and arguing in good faith, IE: they'll just say the wildest shit that's blatantly untrue, and people on the left tend to bust out the facts and links to long explanations of why that's false, and they just counter it with more nonsense until their opponents get tired and quit, then they proclaim victory. But if you just go "you're weird, fuck you" and then refuse to follow up on it they kind of don't know what to do with it.
Also, the amount it's pissing them off means it's working, everyone keep doing it!
This is one of the greatest scams that conservatives get away with IMO, not just in the US but it happens in the UK and other places too. Conservatives get in, go hog wild cutting taxes, selling off public assets and throwing huge contracts to their friends, and then as soon as the other side gets back in they find that they have to now balance the books, the conservatives start complaining and saying they're the fiscally responsible ones.
It's literally happening right now in the UK - we just got rid of the Tories finally after about 15 years, and the new Labour government immediately found a £20 billion hole in the economy which they now have to make harsh cuts to sort out, and they're the ones getting criticized for it by the media.