Why do some people view curly hair as unformal?

For context, I have curly hair. When there's an important event coming up (for example: graduation, prom, job interview, etc) I'm always asked if I'm going to straighten my hair for said event. When I say no, I'm encouraged to do so because curly hair is "informal" and straight hair is "classy".

I'm asking this genuinely, why do some people believe that? Why do some people consider straight hair the default in formal scenarios? Why is curly hair percieved by some as unformal?