As an aside, hats off to Google to being able to serve an 11 year old video with no noticeable delay from what must be the coldest of caches.
And remember that depending if you visited with an iphone, or an android, or a smart TV, or a Chromecast, they'd be needing to serve the video with different encoding settings/codecs/MPEG profiles. So for the hardly ever watched videos, they either need to keep transcoded copies in 10+ formats, all ready to serve with no latency for years, or be ready to live transcode.