because its the longest going joke in history.

The same reason they say "Boys Rule, Girls Drool"/"Girls Rule, Boys Drool" or "Guys Have Cooties"/"Girls Have Cooties" in primary/elementary school. Because they can.

It's been a long-running bunch of "dumb blonde" jokes maybe since the book, "Gentlemen Prefer Blondes" was written by Anita Loos (I think). Like redheads are hot tempered and burnettes are sultry. Just type casting but sometimes accurate. There's so many "bottle blondes, redheads and brunettes" that it could hardly be accurate anymore . . . if it ever was.

This question actually made me decide to look for the origin of the "Dumb Blonde". All I could come up with is a Wikipedia article about it. Sounds pretty reasonable, though, regardless of the missing citations: "Like many popular-culture stereotypes, the origins of this concept are murky. The 1925 Anita Loos novel Gentlemen Prefer Blondes: The Illuminating Diary of a Professional Lady (later used as source for a film by the same name made by Howard Hawks and starring Marilyn Monroe) featured the character Lorelei Lee, a beautiful but empty-headed singer. While some look to this as the source for the concept, in fact, it might be far older. Some have suggested that, because Caucasian babies are often born with at least a touch of blonde hair, an association has arisen tying those having fair hair with childhood and youth (and the accompanying proclivities toward naïvité and/or innocence). Also, as blonde hair is often associated with physical attractiveness and youth, some argue that those around blondes may have a tendency to admire or fawn over them, encouraging some to behave in a child-like manner (consciously or not) in order to gain attention and affection. On the other hand, some have postulated that the association is older still, having its roots in ancient times. The ancient Greeks and Romans were fascinated by the fair hair of the Celts and the Nordic peoples and wished to emulate their red and flaxen tresses.[citation needed] People in the Mediterranean area often bleached their hair or bought wigs made from the hair of enslaved Germanic and Celtic peoples, and most notably the highest-ranking courtesans.[citation needed] Due to this association of red and fair hair with harlots, light hair earned a degree of contempt from the high-ranking ladies of society.[citation needed] In Medieval Europe, the upper classes tended to be darker haired than the peasantry, likely due to the period tendency to marry within one's own class and the fact that lower class people were far more exposed to sunlight.[citation needed] Blonde hair was, at this time, often associated with commoners, who were ostensibly deemed less intelligent. Puritans, associating makeup and dyeing of hair with prostitution, forbade the dyeing or bleaching of hair, creating an imprint on dyeing hair that lasted until the 1920s. The "dumb" side could have been a way for wives of adulterous men to reassure themselves about the infidelity of their husbands; to think that their husband's blonde mistress was sleazy, worthless, ditzy, and not very intelligent[1]. Or, the idea might stem from the idea amongst Romans and Greeks that Northern Europeans were barbarians and thus less advanced than Southern Europeans. An interesting notion is that the Scandinavian blonde is often connected to romantic nationalism, and the stereotype of the blond farm girl or dairy maid. In the actual romantic movement, this type crystallized in literature, mainly Synnøve Solbakken by Bjørnstjerne Bjørnson and the character of Solveig from Henrik Ibsen`s Peer Gynt. Although both characters are positive and even intelligent, tradition often gives them the "dumb blonde" trademark, more or less requited." http://en.wikipedia.org/wiki/Dumb_blonde

Stereotype. Just because there's a saying that 'blondes are dumb', doesn't automatically make every blonde person ever born immediately dumb. Pretty much every "blonde" (dumb or not) in tv, movies is a fake blonde anyway.

Have you ever seen Paris Hilton?

Guys are fascinated by blonds so they like to kid about them. I find it all very stereotypical when people tell jokes about blonds. They joke about it because they wouldn't know how to deal with a blond on a date.