Why when ppl have a bad experience with someone Korean, do they deem all Koreans that way?

Personally, I have had nothing but good experiences with the people that i have met that are Korean. Generally, I get along with people from from all types of cultures/backrounds/nationalities etc.,but I have noticed that with some people I have met or that I have seen their opinions online, when someone asked them a question( I'll mention the one I heard online) like "I am going to Korea for the first time how will I be received?" Or" I am black, do koreans like black ppl?" Some people will give an answer like oh they don't like foreigners, especially black ones or the darker their skin tone is or something like that, I know that they are only speaking from their experience, but is it right to taint the mind of someone (sorry maybe taint isn't the right word) who has yet to have an experience in Korea? How can someone speak for an entire cultural group of people no matter how genous the society is? People are all individuals no matter what, so to say that one group of ppl are a certain way based on a bad experience one had or the one has met is illogical. Sorry for the rant, but this is one of my peeves, when one generalizes a whole group of people no matter what culture based on their limited knowlege/ experience. (I don't think this matters much, but I am african american and I bring things up like this because I consider myself to be a world citizen, lol hence my screen name)