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

https://midwest.social/post/14629110

How come Android doesn't have a built in PDF viewer? - midwest.social

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!

GrapheneOS actually has a built in PDF reader. Open the “Apps” app if it isn’t installed. Look for “PDF Reader” and make sure its installed.
I think I might be using the wrong terminology, forgive me. What I mean is, in my case, why does the PDF have to open in PDF reader instead of directly in the browser (like on iPhone)? That’s what I mean when I say “opening things externally.”
It doesn’t need to be opened with any specific app. Android lets you set default apps for different file types (this is determined by the ending file name characters after the “.”). Since you’re on GrapheneOS, you’re probably privacy-conscience and would be wise to stick with the app included with the OS. This behavior contrasts a user who heads to the Play Store and goes with any PDF reader willy-nilly.
The default app is unusable. There’s literally no scrolling functionality. You have to tap a button to change pages