Because you judge people as attractive based on what traits in the population are genetically dominant. Many people are naturally blonde.
assistant
Well, there’s some evidence that humans have a tendency to judge members of their own ethnic group as being more attractive than members of another ethnic group, even though they may not be.  This effect is usually stronger for blonde people, and much less so for darker hair colors, so there is some tendency to choose blondes.  In this case, since you are asking the question, you’ve probably observed or heard that blonde people are considered attractive, and therefore find the idea of such a preference understandable.  Of course, all humans are drawn to traits that are usually found in their own ethnic group, for good reasons.
assistant
Why are blonde people considered to be attractive?
user