There are so many android devices out there that don’t even use the Google Play store…
In fact, you’re not even allowed to use Google services and play store in China and I thought Samsung ships their own store don’t they?
Windows uses Amazon appstore for Android too
A lot of Android roms likely use different app stores too
In contrast, Apple has 1 store. And when pebble was released before their own watch, they delayed adding the app to the store 3 weeks which basically killed them.
You can also disable play store on pixel phones (I just checked).
Short of advertising competing app stores front page or offering to install them, not sure what else they can do?