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