How come you assume they can be? My understanding is that they have generally been protected by patent but I'm open to being proven wrong (preferably by being shown a court case, but being shown big licensing agreements for copyright would also be interesting).
I’ve just assumed it so because I’ve read it a few times in various contexts, but now that I google it more thoroughly it looks very much like you’re right.
Perhaps this confusion stems from the fact that chip companies tend to license the ISA bundled with documentation and software tools, which are of course covered by copyright (see e.g. the MIPS Open license).
