Because, for all intent and purpose, the three of them have never been on a field mission together.
Heck, the three of them have never lived together in close quarter on a permanent basis either.
There is no history of the three of them being together.
Scott is the common denominator between Kable and Rachel, but what personal ties Rachel and Kable have to one another beyond that?
Shouldn't we see Scott being the one mending the gaps between the both of them, who were foreigners to one another until very recently?
Even if you Don't care about Cable, Scott and Rachel did, they actually knew him for years before his execution by Kable - yet here they're bantering with him as if they were part of a mediocre sitcom…
And if I can buy the idea that Cyclops would make the effort to try and overcome this fact after Extermination, I doubt Rachel would - after all, she was willing to avenge Cable, was she not? So when did Rachel decided she was fine living/working with Kable?
Because here, it looks like they are fine with one another.
All in all, the idea that there could be this kind of dynamic between the Summers without us seeing the process leading to it is one that feels very fake/contrived.