Because in some circumstances it is the smarter choice.
Your spouse relies on the marital relationship for their visa.
They can’t unilaterally take the children back to their own country.
They have no strong support network embedded in the new area.
They have no rights to “the marital home”, because there isn’t one. Due to both spouses living as a guests in another person’s house.
It might not have been plugged into the initial plan, but as a continuing set up it offers destabilising features for one spouse in particular.
People can do stupid things. Make awful errors. But they can learn. Especially if they’ve had to do it the hard way. And they can get quite hard nosed when lining up ducks in the kind of row they prefer.
Awful PR risk for a spouse who doesn’t want to appear cold enough to have engineered regaining the upper hand and then some.
But also awful PR risk for a spouse who might quite like to flee, but doesn’t want to risk being thought of as a parent who would leave their children in another continent.
Will take time to see how it shakes down and who takes the bigger reputation hits.
But borrowing a house makes sense to me given their differing nationalities and the legal baggage that comes with that.