Maybe Gibson sold bass pickups separately sometime in the pre-Henry era, but I can't remember a time when they did. The TBird probably sells far more than the models that used the mudbucker-size TB+ and they never sold those separately.
Gibson isn't alone. Some companies don't want to be in the parts business like Fender. At least Gibson sells some pickups and controls for their guitars. Try buying anything like that from MusicMan. They'll exchange part for defective part under warranty but that's it.
I certainly see more SG Basses around than Gibson TBirds, but you are no doubt right.
I know you can't buy TBird pups either. That was part of my point.
Yes I know about EBMM. R!ckenbacker are useless too and can be extremely unpleasant to boot.
But really is that any reason for Gibson to not support their products?
Imagine a car manufacturer not selling parts for a car that is still in production.
Anyway I have a spare so I don't care now.