Because it's not necessary. To each their own and change yours as often as you like, but sticking to the longer intervals won't make your engine/Trans/primary blow up.

 

Nope!
I won't change my tranny oil until it's gone 10-15k (kilometers) or until it's black, whichever comes first. It doesn't heat up that much and it almost always looks like new oil when I drain it.
Primary is as different story. It gets changed with the motor oil or on every other motor oil change depending on how it looks. On my first few primary oil changes, it looked worst than what came out of the motor. The last time, not so much.
Motor oil gets changed ever 5k kms.