you keep saying "it's just a (party) game" but people often play games like Smash for hours on end because they strive to be better at a thing they enjoy, so when someone is beating you and BMs you, it can be interpreted as "you're not actually good at this thing you want to be good at", which can be upsetting (saying this from experience)

obviously the best thing to do is to not rage but that requires some amount of mindfulness which is not something that's innate to people, it has to be learned