Visually you are absolutely right, most default planes look better than payware ones (not all, but many), however the difference is in the depth of the systems.
It is not the same flying the default a320 than FBW a320, they feel like two different planes, and that is what simmers are looking for.
The DC6 for example looks really good, and while some 2d textures are used most of us consider the plane a masterpiece because of its flight model and the realism of its systems.