From my experience - that's mostly because courses are useless for career. So attending them is an optional, low priority activity. If you could actually get a

From my experience - that's mostly because courses are useless for career. So attending them is an optional, low priority activity. If you could actually get a respectable degree or skills on a CV that matter for anything other than junior-most positions (if at all), you would see higher completion rate. The quality of material itself is often pretty good. That varies from course to course of course, but I must say I started understanding electronics only after an edx course despite having it in my university.