I dont know! Is it a reminder of their percieved unhappiness? Is it a path of least resistance? Is it self abosrbtion? Is it shutting out anyone who is judgmental?
In my case, I can see it as a bit of everything. My son is so laid back, he would fall over. He probably just does not mention anything about the situation, therefore he is not voccaly judgmental. Wife is so self absorbed, probably always has been, so selfishness comes naturally to her.
On the other hand, Daughter has expressed her contempt at the situation, is judgmental, and she has been squeesed out.
I have observed wife trying to hug the kids, as with me in the early days, and it was so false, and just going through the motions.