Partially. You know, your PC has BIOS/UEFI and PCIE busses and other ways for kernel to discover what it is really running on, which drivers to load and so on.

Partially. You know, your PC has BIOS/UEFI and PCIE busses and other ways for kernel to discover what it is really running on, which drivers to load and so on. ARM devices have none of these (except for servers, where Redhat had enough clout to say, that if your ARMv8 server has no UEFI, it will have no RHEL). Not only they add up to energy consumption (it is extra hardware, after all), they add up in the BOM as well (again, it is extra hardware). So we are stuck with kernels targeting only a known hardware (which today devicetree describes to some degree) and where a poke on a wrong port could mean that the entire device hangs. And after all, no consumers base their decision on whether a third party can replace the operating system, so why make a worse (=worse battery) and more expensive (=more BOM) than the competition?