Taking your video example it also just shows that browsers are different: there's tons of video content out there on the web that browsers can no longer play, since they relied on plugins to play and plugins are dead, like RealVideo, WMV, MPEG-1/2/4, etc. As always, ffmpeg is a treasure, though.
There's a whole world of patents that make video a particularly problematic space for browsers, but just the basic philosophy of continuing to work with old static content forever isn't that strongly held (and really, isn't that strongly held in much of software: consider opening really old Word or WordPerfect documents just as an example).
