Sure but the context here, whether you agree with the notion or not, is that at least republicans would actually see Cruz as preferable to trump and they could have potentially made Cruz happen if they banded together. Otherwise, what even is the point of this discussion?
I mean, that is a case of Cruz failing to get an ally... And presumably even one of the more accessible mainstream republicans for him at that. Him almost getting good relations with one of the more accessible republicans for him at that point doesn't exactly make your case.
You are underestimating the hatred cruz' fellow republicans feel for him. But sure, hatred alone is not enough of a reason. But that gets us to the real context here: Establishment republicans did not see cruz as a meaningful improvement over trump. Ted cruz is a hateable turd that has been reviled by mainstream republicans for years and years. The problem once rubio left is not a lack of unity on the party, it's that there simply wasn't anyone to unite against. There was nothing to gain for the party in uniting cruz to defeat trump. cruz isn't one of them, never has been and probably will never be one of them. Even for republicans choosing between trump and cruz is basically choosing between a turd sandwich and a shit sandwich.
It is an example of , at some point, Rubio being interested in pursuing a relationship with Cruz. But that would contradict your idea that it is completely impossible for someone like ted cruz to get someone like Rubio to endorse him.
Whether somebody is hateable is not insanely relevant in politics. It depends on what that translates to. If you are a republican and hate ted cruz, but cruz makes a very good pitch to you for something that benefits you, just like trump does, you are probably going to accept his pitch. Cruz was a hateable man before, yet, at some point, Rubio and Cruz were mutually interested into each other.
You say there was nothing to gain. Yet, then why endorse cruz in the first place? And why did Kasich and Cruz do a mutual pact to cede primaries to the other?
It was half assed, but still, you do not just do a political endorsement if you do not care, because you are weakening your standing in the party by this if you lose.
Kamala Harris quickly endorsing Biden after she stepped down enabled her to become VP. Had she endorsed Sanders, she would not have got into the cabinet. The same is true for buttigieg. Rubio getting on the bad side of the trump crowd is a move that does not benefit him. If Rubio did not want to see Cruz significantly more than trump (no matter personal sympathy), he would not have done that.
One reason Rubio gave for not endorsing Cruz more (to aides, not to the public) was that he did not want to help trump with the narrative of 'two senators banding together against trump'. But, since trump very likely won at that point already, that also obviously prevents more damage to rubio if he endorses cruz in some alliance and still they lose.
But that all still does not change that Rubio wanted a person like Cruz significantly more than a person like Trump.
It also does not change that a killer pitch is a killer pitch. If Someone like Kasich could decide whether he wants to lose to trump and be out of the spotlight because the party gets taken over by trump#s army or whether he can work in a cabinet spot or another important spot in a cruz government, he would decide for the latter.
It is political skill to convince an opponent who does not want to concede with a pitch like that, and a more competent cruz could have done that, just like biden did. No matter how much they hate him.
Biden was able to instantly make people like yang endorse him, even tho yang is farther away in policy matters to biden than to sanders.
Ted Cruz might be bad at this game, but if Joe Biden was not very, very good at this game, he would likely have faced a brokered convention or, in worst case, even lost to Sanders.
But you could ask all the questions you asked before about biden too. Why would Buttigieg and Klobuchar stop their run and endorse biden? They could have gone on, they had more delegates than kasich and rubio at that point. They did not do that because they love biden so much. At least as far as buttigieg is concerned, he got something back for that (and proceeded to campaign for biden in the general).
Why did challengers like Yang and harris endorse biden? I don't know what he did, but his campaign surely approached them with a good pitch.
As i said: A marco rubio who thinks that he lost but could be VP under Cruz for a later run woudl certainly decide for that rather than to have trump win and have the party taken over by the trump army. And it was the lack of political skill of ted cruz to make him see that vision, while biden was able to make someone like harris or buttigieg see what is in for them and that it can succeed.