If these businesses would grow more sustainably (i.e. slower), they wouldn't need such large sums of money to operate. They wouldn't over hire at sales / marketing / engineering / design / operations/ literally every role. In turn, they would be forced to set rates that can cover their actual costs while being a good business deal for restaurants, as they'd have to be around long enough for the delivery company to have any real growth.
There should be economies of scale wrt. a centralized delivery platform that services all kinds of restaurants. The fact that, say, Dominos has been offering delivery for _decades_ means that it's absolutely possible to have a sustainable national food delivery business on $8 medium pizza deals and $4 delivery charges. The tech delivery companies are just plain greedy: I surmise it's their quest for "f u" money that kills their business model right off the bat.