Well, there's also massive scaling, which the average web developer isn't experienced with. Then there's creating a consistent experience across various devices and browsers.

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...

Youtube is a monopoly in a way Google is a monopoly. Just like how there's Bing and DuckDuckGo but most people use Google, most people use Youtube although some do use sites like vimeo. If you think it's so easy to build a site that competes with Google or Youtube you should go ahead and build one and overtake them. After all, nowadays everything is open sourced, you can even build your own search engine from scratch overnight.