DEVs Goal might not be making a game fully? Maybe like me for minecraft starting something new is the most fun? Also ould be they are new to the craft and each time they start a new project their code/skills become better and they don't want to revist old projects to clean then up rather than moving forward. Just my thoughts :)
P.S. Game JAM might just be their thing too, maybe the competition and time crunch is a driving force. Maybe they attend for fun/vacation but they don't have time to dedicate to these projects after the fact?