why did racism ever exist??

I srsly dont get why these ppl couldnt just not have their useless opinions, they can’t help that they were that skin tone. did they think that their opinion would have an impact on someone’s body???  and the fact there’s still racism out there disgusts me because they killed people of color for no in particular reason. it just makes no sense at all because those racist people back then would probably change their mind if they saw how we treated them now. just my opinion though!