Do you have any prejudices?

[festive1]

This. Everyone has their prejudices, whether they recognize them as such, or not.
The challenge is to recognize prejudices for what they are, unfair biases of an entire group often based on individual examples. And ultimately to overcome them.

My main recognizable prejudice? Southern accents. In conversation I'll often assume a person with a Southern accent is less educated and less intelligent than someone without a pronounced accent. Of course this is entirely biased and often incorrect. When I catch myself doing this, I try to mentally check myself and give the person back those automatically deducted IQ points. Nobody is stupid until they prove themselves to be such, and even unintelligent people can have something important to add.

Who says that we need to do so?

Prejudices are part of human nature. It's only contemporary morals that cite they are wrong. Go tell people 200 years ago that prejudice is wrong, and see what they say lol..

My first response was eaten by the internet

I'll reiterate:
One can't apply modern morality to historical figures. We are all products of our times and are limited by such. It is unfair and dishonest to hold people from such different times to one particular standard, it's better to compare them to the morals of their day and see how they fair.
Prejudices are human, you are absolutely correct. To me this raises the question of why? Why were prejudices helpful to humans? I can see them being helpful in prehistoric times, when let's say a rival clan massacred your people, so you shun all outsiders and focus on your own group. That's damn helpful if you've got a biologically sustainable breeding population, instead of engaging in outright warfare, which I'm sure was practiced oftentimes too. But not so helpful in today's modern, diverse, socio-cultural context.
There are many impulses our brains have, but humans should attempt to rise above mere impulses, at least most of the time.