Not that I really back either side of this arguement..but bob pretty much made it clear when he said "Well part of it is typecasting."
You said look at the roles these starlets are taking..well, more then likely that's because they're the only roles they're getting. For example, can you honestly picture Lindsay Lohan in Lost in Translation? I'm sure you can't, and I'm sure the directors couldn't either, so I'm sure there's no chance in hell they would offer her the role. So it may not be so much as not taking the roles...as not being offered any other roles.
And in the cases of Lohan/Duff, they're "Disney products". What other kind of movies are Disney going to make? So unless they're doing a voice or something for a movie like the Incredibles, don't expect them in action movies..and dont expect the 'dramas' to be anything different from A Cinderella Story and other movies of that nature.