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At a guess, it's because humans tend not to think as much as they should and tend to assume the best of themselves when communicating with people.
I.e a non racist privileged white guy may well not consider for a moment that anything they say might be racist.
What would be nice one in a while, is for one of the guys who do something like this to say "fuck me, what I said was really racist and I'm extremely sorry for saying it" rather than try to claim that it wasn't racist.
Tis possible to say or do a racist thing without being a racist I suppose.