Because it won't work. there is no other platform that has the amount of traffic, content or ability to contain said content like youtube. we're talking millions of people and and videos here, no video based site is big enough for that to happen and it probably never will.
This isn't like Myspace where people just moved onto to twitter or facebook because they were surperior social media sites at the time, it's easy to leave behind an old social media account and get a new one because people don't constantly go back and read your old tweets, they only care about what you have to say right then and there, not to mention most people aren't making a living off of posting updates unlike when they make videos. that's why simply moving to another youtube type website simply can't work.
Youtube is sadly unlike any other site in history in terms of scale when it comes to video sharing and it makes much more sense for people to want to fix it rather than risk their livelihood any further by jumping onto some new streaming site that will likely never take off.
And besides, even IF a new site like youtube showed up and everyone decided to jump ship, what makes you think that site will treat it's users differently by not catering to the companies behind all the ads? Because judging by how shitty other big websites treats everyone, I think it's practically guaranteed.

If you are not buying or selling something on a free to use website you are the product being sold. If you don't like a service you don't pay for, don't use it. I wish more people would take that advice, more so with Facebook but it still applicable here.