Take poor Jim Carrey for example. But at least male movie stars can build a body of work out of comedic roles. Carrey, Eddie Murphy, Bill Murray and others have gotten steady work in comedies. But few women do more than a couple of comedy roles in their career. Try and think of film modern actresses who have built their entire careers around being funny.
Actresses have a hard enough time being taken seriously in still male-dominated field.
But if they happen to make their living making others laugh, they run the risk of being totally written off. Look at Meryl Streep. The sharpest example is Cameron Diaz. It seems Hollywood just doesn't know what to do with beautiful, funny women. Maybe it's because people think it's easy to be charismatic and charming when you're as beautiful as Cameron Diaz.