your economy of scale only works if you have multiple models using the same motherboard.
with mobile devices, every model requires a complete redesign of the circuitry, so you only have your scale match the sales of a particular model
as for motherboards including every possible accessory, that just isn't going to happen.
at the low end, it won't happen because they won't want to spend the money on things they don't need.
at the small end it won't happen because all those accessories need connectors, and there isn't space to put them on tiny devices
As proof that this isn't true, look at the fact that most mobile devices don't include memory card slots. That's something that many people want, but the cost (in money, space for the connector and slot for the card) means that most devices don't contain something that's so useful.
for that matter, Apple couldn't fit video on their new iphone connector and resorted to using wifi to cover the 3 inches to the receive built in on the wire. Who thinks this is a good design?
there are far too many possible accessories to put every one of them on every motherboard.