say they sink a lot of resources into maintaining their respective stores
That may be true, but here in France (the EU?), I cannot purchase digital downloads (music, for example) from either Amazon Music or the regular Amazon app.
Why? Because Google (*) want their 30% cut, for a purchase that they had nothing to do with, hosted on infrastructure that they have nothing to do with, and not even supplied or maintained by them.
In essence, "it's digital and it's on our platform so cough up a third" appears to be Google's (*) attitude.
The sooner somebody puts the boot into this, the better.
* - I don't imagine Apple is any different, my experience is with Android.