There's a lot of truth in stereotypes. Not all women nag, but more do than men. Not all men are irresponsible, but more are than women. Since it's very difficult to make statements like that seriously in modern society - usually, you can only say it either anonymously or in groups of close friends whom you trust to not take it personally - a lot of people embed it in comedy, where the filters are lower, and where there's more reason for it to come up in the first place than just expressing bias.
It's not a harmless practice, of course, but it does provide a useful safety valve sometimes.
I seriously doubt that most people who make up jokes or stereotypes truly have enough data on hand to reasonably support even a generalization of this nature.
As Multiheaded added, "Personal is Political" stuff like gender relations, etc also may belong here.