The only answer that makes any sense to me is that they are running on the off chance that Trump dies or is otherwise completely incapacitated (like in a coma) before the GOP primary finishes. That explains why most (all but Chris Christie) are not even criticizing Trump. They are not running against Trump! If Trump can run, they will certainly lose, they all know that. So taking out the front runner is not an option for them. They need the front runner to be taken out by someone else.
Trump is 77 years old (not terribly old in this day and age, but old enough that mortality is a serious consideration). He has terrible eating habits and doesn't really exercise at all. The odds are against him dying in the next year, but there's still enough of a chance that the possibility is on the table.
And if Trump were to suddenly die, I have no idea who the GOP nominee would be. DeSantis seems to be faltering, but that's partly because he can't pick up any hardcore MAGA voters because they are too attached to Trump. Would DeSantis be their second choice? I have no idea. Someone should do a poll of the GOP nominees without Trump, just to see what it looks like.
