YouTubers have had [cringy] movies made of them (Fred, Smosh) or books about their careers and lives (The Amazing Book is Not of Fire [Dan and Phil], A Work in Progress [Conner Franta] and Binge [Tyler Oakley]). They also sell merchandise (mainly clothing).
YouTubers have millions of fans sometimes, and the fans like saying that they are fans of that person!
The books they write are generally about the YouTuber growing up, why they started the channel, and how the channel grew. Old fans love thinking about those times and new fans can learn about the person!
Today's generation doesn't believe in looking upto someone they rather believe in relating to someone and that is what youtube provides them. YouTubers are someone who has the same story like us, who look like us talk like us and that's what creates a connection between a YouTuber and his audience. People feel less like a fan and more like a friend and that is the reason people feel compelled to buy merchandise of You Tubers.
It also gave them a feeling that they are helping their favourite You tubers and second they feel more relatable because they are using stuff which they think their YouTubers are using.
Unlike TV and Movies, Youtube is a more personal medium. After a point, they develop a strong feeling for their You Tubers. They consume so much of their content that that start believing that they know the person inside out. That feeling force them to do anything their YouTubers ask them to do. That is why nowadays brands are approaching these Youtubers to sell their products because they know how much influence they have in normal person's Life.