This has nothing to do with ARM. Unlike the PC there is no standard for system architecture (including boards). ARM based designers like Qualcomm, NVIDIA etc can choose to have their own board and system architecture. For example, they can boot of any memory location or lock the user out of using an exception level like EL3/EL2 etc. That is why the ecosystem looks closed. Nothing stops a SoC designer from giving complete access to hardware.

BTW - any SoC designer that uses RISC-V can do the same.