Youtube is an interesting choice of example, since in a way it's "just documents" - those documents happen to be video, and are surrounded by hyperlinks to other video's pages. The real achievement here was cutting through the intellectual property thicket so that everyone could have an in-browser video player in HTML5.
The first site I ever saw that used Javascript to produce a useful application rather than annoying frippery was Google Maps.
(Rhetorical question: why does everyone use youtube rather than hosting videos on their own sites? Unpacking this question will show the obstacles that "redecentralisation" faces)
>why does everyone use youtube rather than hosting videos on their own sites? Unpacking this question will show the obstacles that "redecentralisation" faces
Two reasons: Bandwidth and Speed. Google has contracts with most major ISP's for caching Youtube videos. This gives them access to the best CDN money can buy! No matter how awesome or powerful your current datacenter/CDN is, it still wouldn't be able to match the fetch-straight-from-the-ISP performance. Interestingly this also creates a hidden monopoly since the barrier to entry is costly.
It's more than that. There's also legal liability (very few people would be comfortable hosting the amount of pirated content that appears on YouTube, nor would they have personal bandwidth for DMCA takedown requests), discoverability (YouTube recommends content you've never seen before, intelligently...by definition a personal site can't do this), and search (which only works when you have enough content to be worthwhile to search over). All of the algorithms used for the latter two points require a good amount of data, so Google gets large economies of scale there.
The first site I ever saw that used Javascript to produce a useful application rather than annoying frippery was Google Maps.
(Rhetorical question: why does everyone use youtube rather than hosting videos on their own sites? Unpacking this question will show the obstacles that "redecentralisation" faces)