The other part of it is that android has been poorly designed for the tablet interface. For an OEM to try to make a tablet out of android they would have to invest significant software development resources into what would amount to be a fork of android (compare Amazon Fire tablets). While they likely have some software developers, they probably don't have enough to maintain that amount of development on android (and always being behind with Google preferring phones).
... and even if they were close to parity with Android (and the challenge that would imply at trying to keep their devices up to date - with the largest investment customers would expect them to last longer) you would still have difficulties with the developers for Android aren't making things that fit well in the tablet form factor (Amazon was trying to bribe/incentivize developers to write for its form factor - https://readwrite.com/amazon-give-developers-rewards-build-f... ).
All that means that unless Google wants to do it and you're willing to go head to head with Amazon its probably not that good of an investment of time, reputation, and resources.
The iPad, on the other hand was the original "this is what we want". The iPad was the first thing that Apple was trying to make.
> Work on the iPad itself actually traces back to 2004, when designer Jonathan Ive and others crafted a new tablet prototype. The product was originally supposed to ship before the iPhone, but the company came to decide the latter was more important, premiering it in 2007 using similar technologies.
( https://appleinsider.com/articles/18/04/03/a-brief-history-o... )
So from the very beginning with the iDevice, the idea of different form factors and sizes was part of how it was written rather than as a phone that grew bigger.