It's because it's incredibly tribal.
Also, it's historically a sport for the "commoner" and as such sometimes pulls in the less educated members of society that have their main influence as Terry the holocaust denying Nazi down the pub that likes to take a bunch of bananas down the match on a Saturday afternoon.
When I used to stand in the Stretford End there was a Chris Smalling chant that was centred around the colour of his skin sang by the home fans, I also understand Romelu Lukkaku had the same thing last season I think, but then I haven't been to OT for a while now so can only go off reports. It was cringey as hell, but everyone was joining in. The education needs to be hammered into these people as the culture is just wrong.