I’m with you mostly, but note that flashing a GApps ZIP is an unofficial way to get the Google Mobile Services. We (as Fairphone owners) are not companies, so we are blind about what is the official way to integrate the said GMS to a developing Android system. It could be more complex than that.
I just mean processes are different, and Fairphone is not able to just flash an unofficial ZIP and redistribute that OS.
Do not forget: Android is a Google walled garden, and Fairphone depends on them (for better (sells) and worse (freedom))
I guess the reason is that the stock recovery is part of Android too. No pure Android, no GMS license.
Also, these previous comments of myself on the forum could bring more info to the matter. We can wikify the topic and add list of exclusive differences between the two OSes, maybe?: