Ask HN: A way to record any video on the web?

There are many "recorder" program on (say) osX, which can take the "loopback audio" and video and record it conveniently, why is it so hard to make an "os-level" / "chrome-level" plugin which would allow recording a video on any website? (and not just have to "reverse" the source of the video and download it, like youtube-dl does)