Last generation, Sony was pushing for it and MS were balking. Because of exactly what you said. The higher selling system doesn't see any need for it.
The funny part of this statement is the XB360 outsold the PS3 ~2:1 in the first few years. The later years devs caught on that the PS3 was a very capable machine and it eventually caught up to (or depending on source, outsold) the XB360. The last generation wasn't just the little guy asking for a part of the big guys multiplayer fun to try to push more hardware.