Venturing to guess / IMO:
Seems risky for a startup "integrator" (hypothetical company who would solve this) to have their business depend on providing specific integrations per startup + POS system. You would need to keep tabs on any changes ea. of the companies like Slice, GrubHub etc. made from both a service standpoint, and a hardware standpoint, right?
To top it off, it would just be one more layer ontop of this, so how would the company make money? Doesn't seem to solve the Pizza guys problems.
Or perhaps you just generally meant, why hasn't someone built a more modular POS unit that would work with any of these guys. That I have no idea about.