It happens too often especially in the FOSS world. How will people turn to FOSS if the developer assumes that everyone knows about REST APIs? Documentation is not an afterthought, is a crucial part of any software.
Developers give away their time and brain power for free. Apparently technical writers are less inclined to do so. It's not like developers are unique. Lawyers do probono work, doctors and dentists volunteer at community clinics, accountants offer free tax prep to people in disadvantaged communities, etc. Maybe there are people who would be willing to take developer documentation of FOSS products and turn them into user friendly documents. Pretty sure that already exists for some projects, though some of that might be paid work (which sometimes is true for development work too).
Developers are taught to document code, not programs, very different skills. Only a small part of the FOSS world has the resources required for hiring someone to write documentation, the rest of FOSS has to rely on the community and hope someone else does it because dealing with the code takes up their time or they just wrote it for themselves and shared it because others might find it useful or have very little time they can dedicate to something which does not put food on the table, etc etc etc.