It seems strange that there isn't any mobile equivalent to the Raspberry Pi, CHIP or the Pinebook (https://www.pine64.org/?page_id=3707) which is priced at under, say, $100, and specifically aimed at people developing their own mobile solutions (OS development and such). It seems like there are no screens (at less than 5") that are available with reasonable resolutions, either.
There seems to be a relative abundance of expensive options (neo900, librem 5) but virtually nothing lower end or cheap.
Is there some reason why devices like this don't exist that I've missed? Is it just more niche than I'm giving it credit for?