Economies of scale. If they sold hundreds of thousands ESP32 or Arduinos every day, their price would likely fall down to €1 per complete board.
> why aren't we seeing feature-rich dev boards based on the same platforms ?
Because those platforms are closed as hell and their manufacturers have no intention of publishing enough technical data to allow developers to write Open Source drivers.
That's the reason why the engineers at Pine64 had to design the PinePhone from scratch to make it run a real Linux distribution instead of using already available proprietary boards that would indeed be a lot faster and cheaper, but also would be restricted to run only Android with closed device drivers and the same crappy untrustworthy software that plagues cellphones today.
One thing that is mentioned by ~everyone who tries to reuse cheap phones as computing devices is that the phones are basically not meant to run all the time and fail very quickly when you try to do thing with the screen on for hours and hours at a time or otherwise hold it to high load.
I am sure there are ways to get around this with some cases but it does feel like there are practical issues with the cheapo/used phones. And that's not even getting into having to wrangle the OS.
Maybe someone can make an Android fork that is basically "make this phone a computing device that should last a long time" that you can easily throw onto phones.
why aren't we seeing feature-rich dev boards based on the same platforms ?