Because outside of deploying your app as a web application there aren’t a lot of other ways to get your app onto Android devices aside from… developing Android apps.

So make a PWA.

Let's not forget, AOSP is still maintained by different individuals aside from Google. Google is mainly interested in integrating it's own "GApps" ecosystem, which introduces ridiculous amount of telemetry and degrades the security of the phone

(I never participated in the AOSP project or read the commit logs, so this is just gathered from news)

My impression is that big features come directly from Google, although I do know that OEMs contribute things here and here, like Sony "donating" LDAC upstream.