I thought of the MEMs I know, and there are many once you know how to spot them. They want someone to mother them because they never had mothering, only smothering.
But as the strong person in the marriage you take care of everything. car problems, child care, household, birthday gifts, social events, friendships, and more. Being the strong one is exhausting. And MEMs will still find things to complain about, all part of keeping you at a a distance.
Instead of helping, the MEM and his family, as does society, tells you to be tough, to suck it up, to move on, to keep it together. Being numb is seen as tough in society. I believe every spouse of a mem gets to a point where they are too exhausted to care anymore. Especially when the children are older, no reason to be strong and tough, it is OK to show vulnerability. And with that, it becomes finally OK to leave.