That and you need to sign up if you just want to have a (short) peek at the content. One also picks up more than one book in the bookstore before buying, if any.
Also, the real value in these courses is that they are the real deal. They should be hard, and thus not for everybody.
Yea, my dropout rate is around 80%. I'll sign up for 4-6 courses that sound interesting, watch the first few lectures all of them, perhaps do the first homework, and then decide which ones (if any) I feel I have the time and interest to complete.
Also, the real value in these courses is that they are the real deal. They should be hard, and thus not for everybody.