I suspect the important difference between the ARM arch and the x86 arch is that there are gobs of generic x86 systems out there with the same exact cpu in them, and with a roughly equivalent basic bill of materials.
In the arm space, there are loads of custom parts. Custom CPUs, custom boards, etc. Very little is the same across multiple devices, which makes it very difficult to have a generic platform to bless as your standard. In fact, there are still ARM chips in heavy use that have no MMU.
This can most easily be seen by building a defconfig for an x86 system and booting it and trying to build a defconfig for an ARM device.