Ok, interesting. I don't think I've really reflected too much why I feel this way before posting (hello internet!). I also may have probably over-generalized ;)

In germany, if you study at a university, you are completely on your own. Nobody is forcing you to do anything and you can push your exams into the next semester until they kick you out for studying too long. So, there are some similarities to online-lectures.

I think I have found my strategy for "getting good" at stuff:

1. I need to be motivated: I am currently motivated that I want to understand how machine-learning really works and can be improved. Getting exposed to research is super fun and currently the reason why I am doing all this. What's a good notion of generalization? What's representation-learning? etc. We have a very active community here and I get to know many people from the industry, academia etc. and through my research-assistant student-job I learn how they work on stuff.

2. Don't do it alone & and have a regular schedule: Solving hard problem-sets together is one of the most-rewarding things I have ever done in my life. I even try to do proofs from papers with others in my free time! But also: train with old tests together, do the problem-sets together etc. There are a lot of groups quickly forming during lectores or before tests that I can just join. I don't need to do everything together, but meeting one or two times a week is an immense boost to my dedication and everything is just way more rewarding. I had a few lectures where I was completely on my own and it was way harder to develop the drive I usually had.

I don't think I can really replicate this for online-courses. I'll adress the points from above:

1. A university is a place where there's always something interesting. So many people studying interesting things. When I started CS, I didn't really know what I wanted to do. The intellectual stimulus is better than I have thought it would be. An online CS-course would probably have also thought me about the topics, but I don't think you get to know any research groups and their topics. I don't think this is a fundamental problem with MOOC, because the situation can be different. But at this point in my life, it was quit important.

2. I think this is the deal-breaker. Reflecting on my life, I don't really do that much completely on my own. Most of my hobbies are not something I do alone. I also don't live alone (and I don't think I ever want to). Doing stuff in a team is just a real Value-Multiplicator (this probably reads weird, but I think it captures best what I mean) for me. It's not that things are not fun to do alone ("=" 1fun), but they are way more fun to do in a team ("=" xfun for some x > 1). I don't know whether organizing local study-groups for an online-course is really the same experience, especially if you're taking just a few courses.

Because we are in the real world, there are always exceptions to the rule. For example, I can program alone without any problems. I could probably program for days without anyone bothering me. It's not that it's inherently more fun, i really don't know why. I always get interesting ideas when I am alone and I like to develope them (before speaking with friends about them). I also like to read, but I haven't done this in a while. Some things are not that fun to do in a group.

I want add that I have already completed one and finished 30% to 70% of a few other online-lecture (some where not really interesting after a while, others I didn't have time anymore). So I am not speaking about the entire concept.

I think I am not alone, because this is a very common pattern at my university.

EDIT: after thinking about it: I really think that some things just don't make that much fun alone. And that includes a lot of things I do in university.