Google controls the Android ecosystem (restricts what OEMs can do) and employs various practices to ensure there's no alternative besides Apple (this duopoly suits both of them). Chipmakers essentially focus on ensuring they can sell access to BSPs which meet Google requirements, as there's no real alternative for smartphones. If you're not Apple, these chips are meant to be used with GMS Android, that's the expectation.

If there were alternatives (like on the desktop and for some parts of IoT) it would be more cost-efficient to upstream everything, but with the current situation it's fully up to Google.

When Google wanted them to move all their chip-specific code to modules (GKI), they had to do that. And if someone at Google randomly decided "hey, now you'll upstream this stuff", then guess what? They'd have to follow, it's how this monopoly works.