Another example of why celebrities need to stick with the thing that made them famous

Why do celebrities think that just because they're famous they can venture off into new mediums? Are their ego's that out of check? What business does Felicity Huffman have writing a book about dating? Granted she has a successful marriage by Hollywood standards but that is because her and her husband are both good actors on the same level professionally and not very attractive. What the hell does she know about dating that would apply to any normal person?
For other examples of actors thinking they can do whatever they want see Bruce Willis and 'Respect Yourself,' Eddie Murphy and 'Party All The Time,' Michael Jordan and his attempt at baseball, Kope Bryant trying to rap, 50 Cent thinking he can act, Antonio Banderas, Britney Spears, Paris Hilton, etc thinking you want to smell like them and anything Madonna does besides being a pop star whore.