why do people do this?

For context, there were 4 of us left in the round, me, purple, black, and rose (viper). I had caught rose kill black on cams right before rose reported it. I said in chat that i caught rose in the act, and all purple said was "sorry blue", before the two of them voted for me, causing us to lose the game. In the lobby i pointed it out, and all they did was pretend to apologise, and call it a joke. Random lobbies are genuine torture.