For the same reason Tom Cruise, Mel Gibson, Tom Hanks, Will Smith, Mike Myers, Julia Roberts, Adam Sandler, Jim Carrey, Harrison Ford, Nicolas Cage, Eddie Murphy, Denzel Washington, Russell Crowe, Leonardo DiCaprio, Vin Diesel, Cameron Diaz, Bruce Willis and Keanu Reeves get paid over $20 Million a film.
"Names" = Script gets read = Movie gets financed = Movie gets directed by a top director = Theaters want the movie = DVD distributors want the movie = Cable Networks want the move = TV Networks want the move = Foreign rights distributors want the movie = People go see the movie = A lot of popcorn and coca-cola is sold.
That's why in a nutshell.
A great name is priceless.