In other words: a pedal board synthesizer would be too much integration compared to the multi-use advantage of ordinary modules. Not at least because of the cost and complexity of finding the right pedal board to integrate into it.
With all due respect I might add that the need for pedal keys is also extremely small if you take it outside the organ world, as there are not that many synthesizer players who perform within an organ setup. Of all rather classic synth performers only Wendy Carlos' studio setup comes to my mind. So I think with the most prominent offers by Nord and Hammond, this highly specialized market is already well-covered - also for synth users. And MIDI pedal keys also offer far more flexibility than pedal keys with an integrated synth engine would, since they allow you to control whatever you like with them.
My point is, that's the problem - it's still specialized. There's no need for this, as if it's only for trained organists to use pedalboards. Consider typical electronica/ambient: it most often uses long lumbering sustained bass notes. It would take minimal skill to play these on a one-octave pedalboard, not much more than it takes to adapt to mini keys.
There's no reason for this idea not to take off, except that it's new and hasn't been seriously considered. Synthesists are very much into new things and experimentation. Well, here's something new and worth trying. You guys are fascinated by new devices for triggering synthesizers. Well, here's another device.