It's because designers are just clueless and don't know how things are implemented. The opposite is also true: the parent is practically an alien here for liking design yet he doesn't even see the point in a photoshop over a wireframe.

I think in general people are too specialized. You probably want designers that do a little development and developpers that do a little design. The full on generalist approach is also poor, I've done both jobs in my life and ended up mediocre at both.