Earlier quoted context omitted.
Because it's a monopoly. Come back when Imgur becomes the only (or one of very few) place on the web to share funny images and Gifs. Farewell.
Are you trying to imply that Youtube is a monopoly? Aren't there others like vimeo, etc.? Besides, there is nothing from technology perspective to make it a monopoly. All you need is a server rack and a bunch of web developers who can build such a site, right?
So, sure, anyway halfway competent developer could bang out a YouTube (or Facebook, etc) prototype in a few weeks, but going from there to a reliable, usable service at scale is something else altogether.
You're going to need some project managers, SREs, sysadmins, front and back end web developers, programmers who know about video encoding, content experts, lawyers, etc...