It's relative. They've got low completion rates relative to university courses. They have completion rates right where you'd expect given a more reasonable reference class.

A MOOC is essentially an extended web interaction, not a college course. Very short web interactions which require minimal levels of work often have conversion rates in the 5~20% region. Think like "Fill out a survey" or "Make free bingo cards."

MOOCs require many hours of work, stretched over months.

Physical college courses carry a heavy precommitment to attend with gigantic costs associated with them. They also have a low completion rate in many circumstances, for example when the college hasn't already done an absolutely brutal filter based on a combination of socioeconomic status, IQ, and ability to take complex tasks to completion. MOOCs don't typically pre-filter based on those criteria, which means their completion numbers are affected in the predictable fashions.