There's huge interest. These Youtubers have learnt to prey on the morally undeveloped, over excited, hormonal 9-16 year old range. They end up being easily relatable pseudo role models with a huge amount of fan interactivity and fan service. Most 'products' aimed at any age group under 18 is generally reliant on the laziest of manipulation strategies, because it works. Young people are young after all.
They aren't actually bad people these Youtubers, but in the quest for fame they've come to mirror those they aim to reach out to. You know, unaware and childish. But it's a choice they made and they deserve any bad rap they get for it. I don't know if you're into him or ever have ben, but the then major teenage-friendly Youtuber Pewdiepie realised this and completely changed himself to be a much more mature-themed, cynical and 'real to himself' character. He's one of a few actually.
But guys like KSI, the Paul boys and such, it's all about the fame and the money. Sad thing is, I can't blame them.