Most people who sign up don't actually want a certificate and aren't trying to complete. They are interested in the subject, watching some videos, move on. When you have class sizes with 1000-30,000 students, it's OK if 90% don't complete the certificate since many more did than in a brick and mortar school, yet it cost much less per student to deliver the class.

In online classes where you need to pay to get the certificate, it's been found that paid students have the same completion rate as brick and mortar university students.

https://www.afr.com/news/policy/education/fee-payments-lift-...

> Students in massive open online courses (MOOC) who pay a modest amount for a "verified certificate" are just as likely finish their course as regular university students, according to a new large-scale study of online education.