All the PCs that where born as clones of the IBM PC had in common a standard bus and the BIOS unifying different hardware, with many more standards added on to allow for plug and play operation like ACPI.

SoC and phones evolved from embbedded microcontrollers where each program is custom, there are no standards to even detect hardware or memory size, the bootloader has to do most of the basic initialization and then hands it to a kernel which is filled with proprietary drivers because radio manufacturers will never release them, and the companies involved are too big now to agree on a standard like early PC manufacturers