Er, I didn't write that right. Edit fail. This part:
Too much work. The point is that the devices are very expensive compared to much simpler devices.
The point is that the devices are very expensive compared to more complex devices. A phone has a LOT more hardware. It comes with a LOT more support. It comes with a LOT more software. It's shipped in a nice box with accessories, at least a sim tool if it's got a slot. Raspi has none of that. It's made with excess SoCs, whatever they can get cheap. Every single part of it is cheapass. And you can get a phone with as much RAM, much more CPU, and both cellular and wifi for around twice as much as the top end Pi. It's got a screen, it's got a bunch of storage, it's got a battery, and lots of such phones are not sold in huge volumes. So why is this grossly simpler device so expensive? Answer, people will pay for it.
The cost of development of a raspi is not much different from say an Arduino Mega. It's got a more expensive SoC which does a lot more, but that's still not a very expensive part. It makes the PCB more expensive, but not dramatically so. The community does most of the hard work of supporting it.