Maybe because the pockets in the back of our jerseys hold quite a bit. If we are going on a long trip without someone providing SAG, then we probably will have paniers or some other bag on the frame.

I wouldn't like a belt around my waist. But perhaps for light stuff it's not a issue. Or maybe it's just not the issue I imagine, since I've not tried it. The period I was using a fanny pack, it was strapped between my saddle and backstays on my Raleigh Competition GS. At least until I decided to get a proper bike bag to replace it.