Because lots of software released as FOSS is not made to attract people to start using it without spending at least some time reading and understanding it. Lot

Because lots of software released as FOSS is not made to attract people to start using it without spending at least some time reading and understanding it. Lots of FOSS is written that way, especially corporate PR FOSS, where engagement and other things are important, because otherwise the cost/value calculation doesn't work out yadda yadda. But lots of other FOSS is released just because someone wrote something, and then just put it out for others to use, if they're curious. They don't owe you anything, and spending time working on the onboarding UX, tutorials or guides might detract from what they wanted to do: entertain themselves solving a problem they care about.