How do people afford to travel the world?

I can understand people going on lime one or two week vacations every so often bit there are some people that will go travel for months at a time and some who do years. I don't get how people do this at all. Like I would love to do this it's my dream to travel the world but how can you take that much time off and afford even if your backpacking. And I feel like most of the people doing these travels are in their 20's and I'm guessing are broke from college