I am surprised that Google spends so much time tackling custom ROMs via it's Play Integrity API. If only they paid that much attention to say, curating the Play Store more, it had be much better for everyone
Google doesn't want distributions of open source Android without Google services to be a viable option for mainstream users because that would reduce their ability to extract profits from the Android ecosystem.
While the focus is surely more on OEMs than end users at this point, I'm sure Google wants to keep the difficulty level for end users high enough that it remains niche.