How come Android doesn't have a built in PDF viewer?

Forgive me for this stupid question. I just transitioned from iPhone to Pixel (GrapheneOS) and I'm curious why there isn't a built in PDF viewer like on iPhone? It feels like you have to open things externally pretty often, but I figure there's a reason for that. I haven't used Android in many years and I recently developed an interest for the technical aspects of things, so again, do forgive this beginners question.

Cheers y'all!