Why do we have instructions such as RDRAND instead of an I/O which would gives us similar results?

I'm wondering what was the reason behind designing a CPU specific instruction to generate random numbers?

The Intel processor has RDRAND and RDSEED. The PPC also has an equivalent instruction.

Wouldn't it make more sense to have a separate chip and just do some I/O to get those numbers? It seems to me that it makes the CPU even more complex for a very specialized instruction (most software never use random numbers!) when I/O has been around for ages and should work just fine.