Why?
Because that's what a lot of the public expect - they expect their movie stars to be ageless, and retain their signature look, the look they are known for.
Of course, as they age, it becomes harder and harder to maintain that look, so they have two choices -
1, do whatever they have to do to maintain that look, even if it means resorting to surgery or
2, age gracefully and accept older, less glamorous roles.
The problem with the latter is that there is no guarantee the public will accept them in such roles, and the work may dry up completely.
If they want to maintain the look that people know them for, then plastic surgery is an option for them.
Joan Collins is a perfect example of this, she still maintains that classic look that she has had from the 60's or 70's, but the problem is, the older she gets the more unnatural she looks.
At the other end of the scale was Brigitte Bardot, who actually did grow old naturally, though she had retired from acting anyway.
Sad fact is, is Ms Field had not had surgery done and had not kept up her appearance, you would probably have been even more shocked at her appearance.