Another thing may be that for many people half-way is good enough. I took the Udacity cryptography course and really enjoyed it. I never took the exam because I didn't see the point. Presumably I'd have been marked down as "failed." But I think that's only because their model for learning still includes "we're the ones who certify you." I'd like to see a model that says "here's the knowledge - go as far as you want."