[ofalkaed]

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.