ARM doesn't have any initialization/boot standard enforced on phone manufacturers so everyone is doing their own and never upstreams anything. It's a general ARM problem. PC with some lucky coincidence avoided that fate and we got used to it. RISC-V is in even worse shape than ARM as the fragmentation is even at the CPU instruction level. Both ARM and RISC-V have some discussions/proposed standards but nothing enforced anywhere.

Interesting insights, thanks!

If I may ask: is that a reason to prevent me from unlocking, adding custom keys and relocking the bootloader? It feels like you are talking about the implementation details below that. I don't really mind if it is different, I just want a common API, and it seems like whatever is done on the Pixel phones is totally fine. Why not make that mandatory for all manufacturers?

Nevertheless, postmarketOS manages to run Linux without Android on quite a few phones.