Unfortunately it is a bit more complicated than that. All these phones run firmware, bootloaders, libraries under license from SoC providers, who package components from other vendors under a license themselves. Opening up the bootloader can be done, but two things have to happen: either the phone is crippled of various functionalities or the manufacturer is in breach of license because all the binary blobs become open and can be reverse engineered. No one wants to go through all of this for a few hundred people who are interested in running their home assistant on an exotic device.

Why would they?

They're actively working on closing the ecosystem even more (no more sideloading), DRM features, etc.

Maybe they'd do it for themselves, but they clearly don't want you, the customer, to do whatever you want with the device you bought and paid for.