Density helps if you're delivering a bunch of small things that you can easily batch together. The kinds of things being tried by these VC funded attempts are not where density helps the most as you either have a delivery that requires too much interaction (e.g., groceries with a fixed time slot) or are aiming for speed (e.g., hot food quick delivery). So there's no reason you can't have a postal service in a very equal society as delivering mail and small packages in urban areas is efficient and you can cross-subsidize nationally. Once you get into multi-temperature deliveries in fixed slots and very fast fulfillment and delivery really cheap labor in comparison to the service price is needed to balance the books. Some of that could be fixed by cutting an order of magnitude from the delivery time by use of lockers and other unattended delivery methods but then you have a chicken-and-egg infrastructure problem.
Densities help for deliveries because they're more efficient in denser areas. But there are a lot of personal services that aren't all that dependent on density.