because they are the only devices on the market that allow locking the bootloader after installation
AFAIK on Xiaomi Mi A2 lite if I flash back the stock fastboot ROM (I had it do it once because I crippled the partition images (this phone is A/B partition device)) it locks your bootloader and you have to unlock it again. I don't know if this applyes to MIUI/Hyper OS phones too, because Mi A2 Lite uses near to stock Android.
Some probably use an API of some sort, because SmartTube for Android TV even synchronises your watch history, subscriptions, you login with your google account etc...