It's because of the so-called stereotyping
is a generalized belief about a particular category of people; it can be, for example, an expectation about the group's personality, preferences, appearance or ability.)
People who didn't meet many Norwegians, Swedish, etc. may just believe that all or most are blond haired (because in all their lives they saw some blond sexy Swedish model or some popular Scandinavian pop band). But people who live in places with travelers or visited Sweden, Norway, Denmark, etc. will know the truth.
The same with the Europeans, Americans, Africans, etc. who never visited China, for example, and don't have enough Chinese population around them -- they may have some stereotypes that "all Chinese know how to use kung fu", "all Chinese eat cats and rats" and so on which is far from right (because only a very small percentage of Chinese will eat something like that and not many will like to learn some martial art; even the Korean Taekwondo is more popular in some Chinese cities than the traditional Chinese kung fu.)
Of course, sometimes the stereotypes can be accurate. Like "Most of the Norwegians aren't poor." and "Most of the Chinese do use chopsticks."