They say this because they are frustrated.
People who lash out to what they perceive as "toxic" in a game with their own toxicity are most likely the same people who have some self-control issues. It's not an indictment of them as a person, it's just that they lack the self-control to use the tools in place and instead chose to act out.
For these people, it is easier to believe that other people are the problem. By blaming external sources/other players, they can rationalize their own behavior and keep acting the way that is easiest for them instead of the way that is more appropriate for a game involving nine other people.
In short, it's part of human nature. Some people are just predisposed to engage in "external attributions".