Its comparatively much more open, with interface and protocols specified and open source firmware implementations widely used in the wild. I'm also including BIOs/UEFI in this.
ARM and phone manufacturing is a hot mess in comparison. We're still trying to reverse engineer M series MacBooks and iPhones are off limits. Android is also not open source, no AOSP does not count.
There would be a lot more competition in the space if the hardware had proper specs, like x86 does.
What competition has x86 seen? I’m old enough to remember when there was more than Intel and AMD. When did they last license x86 to a competitor, 30+ years ago?
It’s great that documentation exists, but it doesn’t make for competition. ARM is at least licensing out to more than two manufacturers.