Three Blondes Walk Into A Bar

Anti-blonde prejudice and stereotyping is one of the most common examples of projection. The coincidence of anti-blond jokes and ridicule being so common that they're practically "normalized", combined with the fact that the most common hair color treatment is blonde, is too obvious to ignore.
Where does anti-blonde prejudice and desire to belittle come from, and why do people so adamantly deny that there's anything wrong with it? On the flip side, if having blonde hair means a person is not smart, or unstable, then why do so many people dye their hair blonde?
Why are human beings so fixated on something as superficial and simplistic as the color of a person's hair, especially when it's so easy to change?
What is the emotional issue that so many people have regarding this?