My theory: he's one of, if not the only people in the GOP capable of drawing enough votes to slow/stop the party's actions and record from catching up to it, and the party's sliding out of competition with the democrats...and the party leaders know it.
They don't like him either, but what they like even less is losing their own power, wealth, and prestige...so in the short term at least, Fat Orange Man is their go-to...until and unless an alternative emerges.
The party has tried a similar replacement in DeSantis and it hasn't worked, so it's back to Trump.
The overall direction of the country and the world has been more or less away from the ideals of that party for at least 25 years, and if not for 9/11 and Trump, they'd have been forced to at least make a show of keeping up with the times or be at risk of collapsing as a relevant political bloc.
Once the patriotic wave following 9/11 faded, the party found itself buoyed by the votes and support of a racist backlash to the election of a black president, and rather than choose to take the votes without shifting platform to cater to those voters, instead they slowly but surely steered the party in that direction, openly courting the most backward, hateful, and dogmatic among us.
Conveniently for them, the democratic party put up one of its least popular candidates even as they found a mouthpiece in Trump willing to and skilled at courting these voters and willing to double down on the bullshit he spouted.
Rather than try to distance the party from that nonsense, they've all jumped aboard, and Trump's personality has forced them all to swear loyalty...and now their only option seems to be to back him as long as he wants to play politician, no matter how bad it gets.
Personally, I'm hoping that a combination of multiple convictions and political losses for Trump over the next few years, combined with his eventual death sometime in the next decade, forces the GOP to either wholly reinvent itself, shifting towards/to the American "middle" (still right leaning by global standards), abandoning many of it's worst positions and becoming a more responsible and reasoned platform for conservatism...or that those combined effects cause the party to implode/fracture, finally giving America a chance to have more than two stable, long term, viable political parties.
It's a devils bargain the RNC made when they first championed this brain drain MAGA nonsense.
Now the field is flooded with candidates who can repeat these easy to remember, ultimately meaningless nationalism talking points and the RNC will be split.
Any type of split means the Dems will sweep. The Dems are gonna run one candidate, and it's the incumbent.