Because traditionally, Blender modeling works best on a clean quad-based mesh. Just look at any modeling tutorial for Blender and one of the first things you learn is to always keep a clean, quad-based topology, and avoid triangles and n-gons as much as possible, as it will make further work on the model more painful, if not impossible. That starts with simple stuff like doing a loop cut to things like uv-unwrapping and using the sculpting tools. It's also better for subdivision surface modeling. You can of course use tri-based models, but if you want to refine them manually, it's often a pain. Usually, for me it's pretty much a "take as-is or leave it" situation for tri-based meshes, and since I see these AI-created models more as a starting point rather than the finished product, having a clean quad-based topology would be very important for me.