[surajrmal]

There is nothing about Android as a platform that forbids installation of your own OS. That's a phone oem choice and the fact Pixel phones can be unlocked proves as much. In fact this is the reason this project is even possible.

[Aachen]

There isn't, but part of Google's requirements for using the trademark "Android" is iirc shipping a locked bootloader. If you also want to provide your users with the Play Store (many people will perceive the device as unusable without that), you also can't ship it with a su binary or something. It needs to come in a locked state where people only get user-level access, no permissions to read the data stored on there (outside of Downloads and DCIM and the like), no permissions to use TCP port 22, etc. Like the level of access many employers provide to non-tech personell as a device they don't own. As to why manufacturers are less and less often adding support for unlocking the hardware, I can only make assumptions

Google is requiring it be closed and leaving the unlock entirely optional. That's a choice

[surajrmal]

Right, because the android security model considers app developers independent entities with security privileges equal to those of the device owner (in that both parties need to authorize access for things to work, the device owner doesn't have more privileges than the application developer when it comes to the application). Those mechanisms are necessary for that security model to work. If you want to operate with a different security model that's fine, but you just need to use something other than Android. The bootloader situation being optional is Google not getting overly involved in the device maker's business outside of the scope they should have influence on. And they set the precedent via Pixel for how they think others should do it.