The video is kind of waffling a bit and not answering the title question in depth, but I agree with the general response. It’s mostly about marketing, which Epic has been very good at for UE. Beyond that, after you reach a certain level of popularity it becomes a positive feedback loop where developers choose UE because it is so popular, which makes recruitment and onboarding easier because more people know it. This in turn makes more people want to learn UE because everyone is using it. And so it repeats itself, until near monopoly is reached.

Without watching the video, cry engine at some point got leased to Amazon who used it to make lumber yard, which star citizen used and modified. eventually lumberyard got modified and made open source, and is now o3de