They are leaving ahead of the elections next year. I can think of quite a few reasons why they might do that.

• Staying means being forced to either defend the indefensible or facing backlash and challenges from the right, either of which could damage any future political ambitions they may have.
• Getting out now leaves room for others to get elected, which keeps them from being held responsible for their party losing a seat.
• They know how nasty things are going to get and they want out before leaving becomes too dangerous. It wouldn't be the first time that Republicans in congress were afraid that pissing of Trump's base could put them in harm's way. That was not helped by Jim Jordan supporters agitating their followers and stirring up death threats against their colleagues just to get their way during the fight to become speaker.
• Because they are getting pushed out behind the scenes and are choosing not to fight. I'm skeptical of this but I can't say for sure that it isn't happening.
• Because they expect the next election to be a disaster and they don't want to get caught up in it.

Basically, most reasons come down to either just wanting out, or wanting to make sure they don't ruin their future political career choices.

If they stayed and voted with democrats, went independent or switched parties, or tried to start a new party, most likely they would end up just as unemployed but with fewer friends and no followers. Going directly against the party would lead to the party itself attacking them, along with the right wing media that many of their supporters get all their info from. Most districts lean to one side or the other, and while a few politicians have made careers on being independent or moderate, the ones that had been on the Republican side of the aisle are all gone, as are those who had the courage to take a stand against Trump when it mattered.

Honestly, at this point they may actually be able to get more done by quietly coordinating with others who have left or been forced out and organizing support for whoever emerges as a viable Trump alternative in the Republican primary. That keeps them out of the cross hairs and at least increases the chances that Trump will lose either in the primaries or the general election if they can at least drag out the fight for the nomination.

If Trump loses badly, political winds could shift in the party. Or if another politician gets the nomination and goes on to win the election, they'll be in a position to push Trump's people out. That's how his people took control in the first place.