Right, but the kernel has to know the layout and structure of all of these structs, anyways, since it has to extract the data to use it. So it's only a little more work, comparatively, to also have 32-bit parse code.
Whereas currently, libc doesn't have to know anything about ioctl formats, it just passes a pointer along. And so if you compile a new kernel module that has some random new ioctl()s, and install the corresponding user programs, everything works. But if libc has to do the marshaling, I also need to update my libc, which is much harder.