Because they have been told for hundreds of years that it is!
Also, most people of the current generation grew up on a diet of television which doesn''t necessarily reflect real life.
Let''s face it, men have ruled the physical and financial areas of life for many thousands of years. For the same time, women have been the primary caregivers. Whilst feminism (which I wholeheartedly endorse, more than most women I know!) has made great strides in levelling the playing field, unfortunately there hasn''t been a corresponding erosion of the view of women as caregivers.
Someone once told me that sexual equality is the idea that the two sexes can become equal by focussing solely on one of them.