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.
We (and our competitors) are partnering up with Grubhub, Eat24, etc. to solve the hassle of multiple tablets at the counter. Orders can just flow seamlessly from Grubhub's web site / app to our software (which the restaurant already uses), and the restaurant staff just has to confirm the order.
Unfortunately the margins all pretty thin, and ultimately you try to make money off payment processing.
My guess is that the companies would otherwise have trouble enforcing their cuts if the customer chooses to pay when they pick up