autofocus - Why don't most cameras use infrared for focusing?

All auto focus assist lights are either red or white. Why do DSLRs not use infrared instead? It seems it would be beneficial to a photographer to not blind their subject before taking a picture.

I've done some google searching, with unsatisfactory results. Some say that the lens "isn't optimized for the wavelengths of IR", I don't know how legitimate that claim is.