Yeah, not doing sequels is leaving money on the table. If you're an artist and you made big money with your novel about sandwiches or a painting of a twister, would any of us have the artistic integrity to not write another food novel or create another weather painting? How many plays about royal families did that hack, Shakespeare, write?
Furthermore, if there's more stories to tell, why not tell them? I don't remember there being a Law Against Art that dictates that the only true art is when the artist tries something completely different.
And further furthermore, if people want more stories of cars or toys or fish... why not?
Besides, Pixar would have had another 'unique' film out in Newt if it weren't for another company's similar Rio. And Coco is most likely delayed because of that other Day of the Dead animation.
Besides besides, the last original, Dinosaur, wasn't exactly an artistic masterpiece. There's no guarantee of higher quality art simply because they do something 'original.'