For the same reason that CPUs don't operate on bignums. ALUs are fixed width, and you don't want to microcode every last instruction.
That said, the original vector processors from the '70s and '80s were kind of like what you describe. Not coincidentally, they died off with the RISC movement.
That said, the original vector processors from the '70s and '80s were kind of like what you describe. Not coincidentally, they died off with the RISC movement.