Because of lack of standardised firmware on smartphone. PCs uses UEFI + ACPI to give information about devices to OS, on smartphone world different chip manufacturer have different bootloader and they load linux kernel and pass DT(device tree) which explains what devices are present and also smartphone were used to be less powerful, contains less storage (in past) so what they do is to compile linux with only the drivers which are actually present on that particular devices to save space. Thats why a smartphone rom is not a generics image which will run on all phone. But android/linux can be compiled like a generics image, look androidx86 project for example.