ARM has patents on various parts of the ISA itself (the same applies to x86: the main difference is x86 patents are well enough split between AMD and Intel they

ARM has patents on various parts of the ISA itself (the same applies to x86: the main difference is x86 patents are well enough split between AMD and Intel they are essentially forced to cross-license). You can't make a compatible version without a patent license at least. (And the ISAs have evolved since they were first made, with new patents involved: you can now make a very old version of an ARM core without a license, but such a chip would be irrelevant for most anything today).