Because they cause. And this was proven by highly educated people.
Moreover, people in games act in accordance with their true attitudes, which are often deeply hidden in real life.
You're conflating association with causation. People acting in accordance to their hidden natures in video games is not the same as video games creating or causing hidden natures to spontaneously exist in people. Those are two very different claims.
To the idea that people behave differently in video games I can agree with nuance. People don't behave differently because of the video games itself, it's being enabled by anonymity and/or the lack of consequences. People often behave with far less empathy and courtesy even in discussion forums such as this one, so it's not exclusive to video games.
To the idea video games cause behaviors in people - no study has never found conclusive evidence proving video games
cause certain kinds of behavior in people. The common claim is violent video games cause people to be violent. If that were true, then I would LOVE to know what kind of video games Genghis Khan was playing in the 12th century - a time period when video games did not exist. What exposure to violence actually does is desensitize people to violence. A person can be both numb to violence and still choose to not engage in violence.