Well you can also look at how attempts by Google and others to upstream went initially. Mainline outright rejected some of the core tenants of what Android was

Well you can also look at how attempts by Google and others to upstream went initially. Mainline outright rejected some of the core tenants of what Android was doing just because it hadn't been an issue on desktop/servers. Things have since slowly changed, but even stuff that shouldn't have been controversial (like ashmem) still took years and when mainline finally added an equivalent (memfd_create) it is just incompatible enough in core behavior that Android can't switch to it.