Advertising psychology is complex, and gender role portrayal changes depending on the product, the "empathy/identification" footprint of the ad's visual thrust, age of target audience, etc. The question described an old-fashioned, oversimplistic view, long ago abandoned by the ad industry. The 1960's saw the transfer of gender roles in the workplace and in language (he/she, etc.), and the ad industry was quick to seem "avant-garde." Today, no-one would depict all doctors as men, all nurses or flight attendants as women, etc. The assumptions about child care, too, have evened out, so that every diaper changed, every meal cooked, every hose vacuumed, etc. is shared by both genders. The paycheck and its distribution are not gender-specific any more, at least not enough to justify the question's assumption.
First of all, it is important to note that men and women are not “always” portrayed in these roles. For example, this ad shows a man playing a role that could be called that of a “homemaker.”
However, it is true that most ads are not like this. The reason is that most men are not the primary caregivers for their children. A very small percentage of men actually act as stay-at-home fathers.
But that is not all. We in the US tend to hearken back to the ‘50s and early ‘60s as a golden age of sorts. This was a time when the US was the undoubted leader of the free world. It was a time when prosperity was increasing and it was a time before (for the most part) the sorts of cultural strife that has made our country less cohesive. Therefore, many Americans think that the social relations of those days were the ideal. In those days, men were the bread winners and women were the homemakers. We wish in some ways that we could go back to those simpler and better (in our collective memory) times. Therefore, ads use images that bring us back to those days as a way of making us feel good about the product being sold.