As I have a youtube account, and am quite possibly the most sane and logical person on the internet at the moment, the reason they're everywhere is;
1.) Anonimity is a hallmark on the internet, and as such, people will release every stupid, idiotic, and moronic parts of themselves in order to cleanse themselves of these "evil" urges before continuing on with their lives. In short, thanks to the internet, people of all sorts can now lead double lives.
2.) For some reason, be it flaw of evolution or some sick, twisted cosmic joke, most people on this planet are idiots.