The apps absolutely already do merge any orders they can. They can't control conditions at the restaurant or on the road however and so a lot of the job is really being a flunky and waiting in line and in traffic which are both quite high around lunch and early evening. It scales very poorly and you probably don't want your order sitting in someone's car for half an hour while they wait for another customer's order.

As far as paying well, I don't think that was ever really expected, but these companies just seem like losers for all but those who managed to cash out in the IPO.