Judging from
1. the content we've had, and
2. the manner in which the feature is implemented on the other servers to which you refer,
it appears there are great difficulties creating the UI elements to facilitate custom MMs. Either that, or the extra files that would be need to be created, or the fact that zillions of custom MM pets might cause everybody's computers to lag to hell and back.
Possibly one of those, or something else. That's my guess.