Well, I think, first of all, because they like to see how ordinary people like me and you achieve success and become famous. When people see how other people start taking steps to make their dream come true, they begin to reflect on their own life and what they've done, or rather, haven't done with it, and focus on what dreams they have and what they can do in order to fulfil them. So, it means that talent shows are a kind of inspiration for many people, something that motivates them to stand up and move towards their goals as such shows prove that hard work and persistence can help people achieve anything they want in life. Well, and secondly, just because people really want to see the hidden side of show business - all those challenges contestants face while preparing for the auditions and interacting with each other. Nothing more, I guess.