Building the tech part of product is not challenging part.

Distribution has always been historically expensive for any content. While digital distribution is cheaper than ever, B/W costs for a product like YT is enormous .

To make unit economics to any sense, you will need to spin your own CDN and buy b/w from other CDNs when that it is cheaper. For your own CDN, you are looking at few hundred PoPs and thousands of peering agreements with ISPs in each geography maybe lease cables too . This is a substantial investment.

Even if do all this you still have to compete with Google on android where they have a distinct advantage, YT is pre-installed in billions of devices and user account already activated. You will have to pay OEMs a lot of money to get the same kind of user base pre-installed and still do your own user activation.

There is a reason why nobody is trying seriously. the next largest competitors DailyMotion and Youku are 1% of YT size.