Because of religion, colors, how we perceive those with different colors, because of politics, because of mis-conceptions etc etc
too many to list.
Like in Saudi Arabia and UAE all the cleaners and low class people are from Bangladesh and i have gotten this misconception that Bengalis are all poor short height-ed people.
Generalisations. There are very rich Bangladeshis, as well as very poor ones. There are very rich Saudis and there are very poor ones as well.
In our school, there are some Saudis. One characteristic we, Bangladeshi students, notice in all of them (guys only) is that they don't seem to brush their teeth in the morning. Their mouth, and body, always smell so bad. Believe me or not, we think that they never brush their teeth! Anyway, these are all generalisations.