Generally speaking, and I’m not talking about your Raspberry Pi’s, but even there we find some limitations for getting a system up and booting - and it’s not for lack of transistors.
But say if you take a consumer facing ARM device, almost always the bootloader is locked and apart of some read only ROM - that if you touch it without permission voids your warranty.
Compare that with an x86 system, whereby the boot loader is installed on an independent partition and has to be “declared” to the firmware, which means you can have several systems on the same machine.
Note how I’m talking about consumer devices and not servers for data centres or embedded systems.