Even AOSP phones come with extra hassles
While some/many Android Apps are compiled in a way that they work on both on Android and a myriad of AOSPs without modification (especialy apps for developing countries like mine were AOSP stuff is more prevalent), yet other Apps are tailored to Androd and a few Specific AOSPs (for instance, my AOSP TV has a bunch of Apps tailored to both Android and Tencent's flavour of AOSP, but no guarantee they would work on other AOSPs TVs that are not Tencent's), other Apps are made in such a way that they only work on Android propper (because they extensively use Google's propiertary libraries), therefore needing MicroG/MindtheGapps* to function properly.
But many AOSP phones and AOSP ROMs ship without MicroG or MindtheGapps to minimize legal risks. They expect the user to be smart enough to decide between the two, and then install the chosen one.
Telling a "normie" user of these Apps to use different App Stores (as the Google Play Store depends on Google Propiertary Services and Libraries) AND seek completely different Apps (and re-learn how to effectively use them) as an alternative, or to decide between MicroG and MindtheGapps and install it (and the Google Play Store) before using the APPs they like (with no guarantee that all the apps will work) introduces friction either way. Not insurmountable friction, mind you, but friction nonetheless.
As a matter of fact, last year, when I set up my new Razr+, I substituded all the Apps that I had from Amazon and F-Droid (on my Android KeyOne) with the same Apps from Google Play Store, precisely to minimize fritcion....
I personally would have no problem (did that a lot in the late stages of the life of my BlackBerry Q10, and I foresee another round with my AOSP TV**), but other people would be less inclined.
* MicroG and MindtheGapps are the most popular compatibility layers that emulate Google's Android closed Source/Propiertary libraries/services. Others exists, and may be more prevalent in the future, but for the time being, these are the biggest ones.
** That particular AOSP TV is so restricted in processing power, RAM and Storage, that for the time being I am making do with a Chromecast second gen, and using it as a dumb TV. I'll delay as much as possible the install of any Apps to let the AOSP ecosystem evolve, so that when I NEED to install something, I know EXACTLY what to install. Experimenting in the TV is frutratingly slow, so, re-working in a few months/years because I bet on the wrong horse, is not something I want to do.