Here is a thread from 7 months ago where more people noticed the video was plagiarized due to a DMCA of a re-upload: reddit.com/…/internet_historians_man_in_cave_vide…

But to actually answer your question, it takes time to prove (or even notice) when a work has been plagiarized, particularly when the person who did wrong does not mention, or intentionally hides the original source. The Hbomberguy video is about exploring in depth and the IH video is just one example, not the main topic.