Why? A simple reason. They love music, and performing it in front of millions of fans would give them the best feeling that nothing could ever beat. They will do whatever it takes to make things right in this world. Especially the people who helped and will help get their career farther into the music biz, so they can spread the word to not only the people who found them in some possible way.
How? Well, let's just say that being famous and rich isn't what they're here for. They're here to do what they love most and that's making music and performing it.