What would have prevented Indians from seeing dino fossils and incorporated those images into their paintings?
Having better things to do. Generally at least. Lewis and Clarke reported their Indian guides being in mortal terror of the Mishipishu/stegosaur glyphs around the Mississippi and the meaning of the things was clear enough i.e. they were warnings, saying "Be careful, one of these things lives close by". I mean, the things weren't just art work for art's sake.
Amerind artists would touch them up every few decades or so which is why they still exist and the stegosaur was long gone by the time a later artist added the horns which you see on the image from Agawa Rock at Massinaw, Lake Superior.