The better question is whether or not this merger makes any sense.
If SpaceX pays too much for it, other SpaceX shareholders have a case against SpaceX leadership. If xAI accepts an offer that is too low, other xAI shareholders have a case against xAI leadership. Given that the leadership is basically the same people, they are very well incentivized to come up with a valuation that is as fair as possible.
And this is not just theoretical, Musk has already been sued successfully once on a similar case, when his companies gave out too much free support to the boring company.
They don't, so why shouldn't he think that way?
Maybe this is a good deal for shareholders of SpaceX and xAI. But then maybe it isn't a good deal for one set of shareholders. I have no idea, but I would love to be a shareholder in SpaceX and would not want to be a shareholer in xAI. Totally depends on the price of course.
Yeah they would totally be doing this if they weren't in dire, desperate need of more money ... pinky promise.