But that is a big part of the problem - these women didn't have ability to begin with. They were always eye candy. In a few years, Halle Berry will have to complain about age-ism as well as racism: hot babes can get away with no talent, but a 50 year old woman (Davis, Hepburn) needs talent.
Here's another reason - movies are made for teenagers, not adults. And here's some more:
Sex is easy. Romance is hard.
Violence is easy. Suspense is hard.
Special effects are easy. Memorable dialog is hard.
Someone promoting a movie-to-be with sex, violence, and special effects can deliver. Someone promoting a movie-to-be with romance, suspense, and memorable dialog can only hope. Since the former can be guaranteed, and since it sells - we get a lot of it.
The problem is that when people are good, they are good in a very wide variety of ways; should you film to indugle that nature, you limit your audience. By contrast, when people are bad, many of them are bad in ways that are highly common and reliable; sex and violence --that's simple!
Sadly, for marketing types, filming bad stuff is a good way of limiting business risk.