Your relating to what you see on the product thumbnail versus what you see on the page entering that product right?
I assume what your telling is right and that you indeed found products having only one format, note however that CGtrader staff converts for you to other formats at request and no additional cost (as far as I know).
Just visited multiple products to get notion of your problem but every single page I visited displays multiple file formats, I found most products to be offering OBJ and FBX.
My impression is that probably small number of products deliver only one format, you probably had the bad luck to have encountered such products?
Also note that products having less formats rank more poorly versus products offering multiple formats, so if you wander all the way back then you probably going to find more products having some flaws?