With something like Udemy, where you have to pay for courses, Udemy needs to review every course before selling them. You need to check them for accuracy (which would mean having them reviewed by experts in the field), as well as verifying the author's identity and doing at least a cursory check for stolen content.
If Udemy /wants/ to be as open as YouTube, that's fine. It just means that it's no better in terms of guaranteeing accuracy, which doesn't make much sense considering you have to pay for courses. If Udemy and YouTube offer basically the same minimal guarantees that their content is accurate and not stolen, why would you ever use the paid service?
Because you have to pay to access courses in Udemy, you may not even be able to tell if a course is stolen or not without paying for it. Obviously, you'd be able to tell if it was stolen whole cloth, but if it's been paraphrased, you'll probably want to be able to look at more of the course to make sure it wasn't just a coincidence because someone else decided to do the same topic. Do you want to pay just to check if it's your own stolen work?
While I haven't done it myself, my understanding is that reporting stolen content on Udemy is unnecessarily difficult, and that Udemy generally takes a long time to take it down. Now, maybe that's not true (or maybe it's not true anymore), but that doesn't reflect well on them if it is/was.