Because it seems pretty implausible to me. Bigger than a giraffe, comically large head and folding leather wings doesn’t scream aerodynamic. This article even gives the altitude at which they “glided” around. I’m no expert on aerodynamics or dinosaurs but with as much as a 36 foot wingspan I don’t see how it could go through a complete flap motion on the ground without the wing tips slapping on the earth.
Just because it had “wings” do we assume it must have flown or is there other proof I’m not seeing?