You can't make an open source platform around a core that requires a commercial license. Had Google done as you suggest, there would be no AOSP, which is fairly strategically central to Android.
Tactical choices are constrained by strategic goals.
> I was talking about third-party OS builders, which could anyway pay for licenses as they always did.
Or they could build their own OS's from scratch, as they always have also done, sure; but an important part of Google's strategy, though, was to make the core of Android open source, which they couldn't do if something as fundamental as the basic runtime required a paid license from a third party. And AOSP is -- empirically -- not useless to third party builders without the separately-licensed Google services.
> End users have no use for open source, besides free beer.
Just-for-me modifications are a real thing, if maybe not all that common, so I wouldn't say "no use".
Tactical choices are constrained by strategic goals.