Why exactly do leafy vegetables turn to mush when frozen uncoooked?
In freezing, it is mostly the bursting of the cell walls as the ice crystals expand.

Much like a water balloon holds its shape when it is intact, but becomes flaccid when punctured, so to the cell walls comprising the vegetable.

See related, for why a similar thing happens when you cook spinach:
Why does spinach lose its texture when cooked?

Cooked vegetables are already soft and their metaphorical balloons partially deflated.  It is not going to get a lot worse for the freezing.  The par cooking is mostly disable enzymes which would change the color of the vegetable to dull olive drab, or negatively affect the flavor.  

See: Why blanche vegetables before freezing?