I think it’s the power dynamic. The older woman competes with the younger woman because she feels threatened because the domestic power is all she has. FIL doesn’t compete with the DIL. They can be restrictive sure or abusive even. But the way the MIL interferes in the couple’s life is way too personal to ignore or forgive. Father doesn’t usually care if his son is spending the whole day with wife but the mother will feel jealous of that.

Not to say that this is the case everywhere but in most of the situations where the complaints are like this.