I like that you brought up Android, because the experience in shipping kernels to billions of devices has led to numerous upstream improvements, as well as an ecosystem of devices that support other Linux-based operating systems. Sony and Apple don't enable this by forking FreeBSD.
It used to be even more comprehensive, which enabled projects like OpenDarwin to exist, but yes upstream can benefit to at least some degree.
And this happens even though there is no copyleft forcing them. Remember that if you retain your own fork, you now also own the maintenance, including aligning your proprietary changes with incoming upstream stuff.
If they do not contribute upstream, they do not benefit from each other’s changes.
That’s their choice. It’s arrogantly coercive and authoritarian to try to use market position to force them to make one choice or another.
After all the modifications mentioned are probably quite use-case specific. Not sure how much this can be generalized