Probably the real reason is that it would increase the cost of the pickups and there aren't enough customers requesting the change.
Mechanically, traditional pickup coils have to be flat. The shell could be made with a radiused top and split coils could be installed in a "roof top" configuration or adjustable poles with ceramic magnets could be used or a sidewinder coil design could be custom fit.
In our case the radius at the bridge on our long scale basses is 25". So flat it wouldn't pay to tweak the pickups any further for output reasons.
Ramps are so personal you'd have to first dial in your pickup heights to perfection, then I'd look at laminating some plastic or ebony tops onto the shells and radiusing them. You're going to need a pretty sophisticated luthier to pull it off though.