I think the key difference is that college is trying to set you up for life, whereas MOOCs are available for life. When I have turned to MOOCs in the past, it was because I had a specific goal in mind. Once that goal was met, which may have been half-way through the class, I had no reason to continue. In the future I may have a new goal that requires I finish the course, and I will do so at that time. There is no advantage to forcing me to have it complete within a set timeframe.