I'm assuming good faith and giving them the benefit of the doubt.
Of course it might be that they want more control. In addition to controlling the world's most popular web browser and the world's most popular search engine and the world's most popular online advertising network and the world's most popular online video service.
It's really hard to when there's already technical solutions. They could require a process like bootloader unlocking that puts it in "dev" mode for instance
While signing is useful, leaving no escape hatch imo is blatantly predatory
Assuming good faith and giving the benefit of the doubt to google is just naivety.
They have shown time and time again that they will take as much control from you as they can.