My PIL have stayed together despite a severely dysfunctional relationship.
Huge amounts of bitterness and resentment, which they are sure they hide from everybody (they do not).
DW and her brother have been hugely affected, although BIL left home at 16, leaving DW as an only child for 10 years. She carries so much guilt for being the reason they were/are both so unhappy.
The OP that mentioned "staying for the children" being an acceptable excuse for inertia/fear of change - that's a very interesting point.
PIL have very traditional split responsibilities in the house, and I'm sure it the thought of being responsible for all aspects of a household on their own is terrifying for them. Maybe terrifying enough not to want to consider splitting.