The hardware processes to fabricate a thin phone are complex. Sure, manufacturers do it every day, but if you are a new entrant starting out with a pick and place machine and an OLED screen, you can expect to spend a few years replicating those processes and a couple million bucks on specialized machinery - just for the QA process. So, a big difference to if you just want to do a board. Neo and Librem are both vapor at this point. Neo looks fatter so will be easier, If Librem manages to ship something decent they will have gotten very lucky with their choice of mfg partner. Their budget is very low.

It isn't that the niche is too small per se, it's that there is an existing if incomplete solution that makes the margins unattractive for the given market size.

Consider purchasing a lower-end mobile phone that is unlocked & rootable & known to run open OS options. Perhaps the Moto G4? Or if you don't mind devices that are no longer supported by the manufacturer, The Nextbit Robin.

Isn't Android open source?

You'll note I said open in the title - I should be able to reasonably easily choose another operating system on my phone, or run something which does not rely on Google to run (as AOSP does through gapps etc)

He's asking about hackable mobile phone hardware (on which, yes, he would probably want to run Android if possible, but also run some other open OS, including his own).