This is because since the beginning in the 80s, the x86 platform and later the x64 platform is an industry standard and everyone who wants can write an OS for this hardware platform, no matter if you use an ASUS or Acer or IBM or whatever other manufacturer of laptop or motherboard.
Smartphone hardware is mostly proprietary including closed source low level drivers.
Providing a standardized hardware platform brought the commercial ruin as hardware manufacturer for IBM, so in present time, I think, phone manufacturers will not follow this way.