Two speakers actually do a pretty good job of creating the illusion of spatial location, because they work the way ears do, only in reverse. Electronic processors based on binaural perception enhance this effect. Extending the theory further, the light emitting display on a synth panel works in the opposite way to how eyes work. In this light, the construction of the piano keyboard is clearly a reflection of the human hand. Like fingers, the keys are arranged in a linear side by side manner.
Fingers push down in one direction and the keys push back.
However, the keyboard is only a digital interface. Each key is just a switch, which can only be turned on to a single amount and turned off sometime later. Traditionally, expression is achieved by creating pointillistic suggestions of a continuous gestural space. What finally occured to me is that what's missing from synthesizers is a reflection of muscles. Continuous muscular exertion is required to play a traditional musical instrument, and there is no analog for this in the synthesis architecture.