Because the group that did it is known.
bq. Fortunately in this instance the perpetrators are super visible.
They are (if you mean the Twitter account), but how does this tell us that the people behind it are gamers? Because they have almost exclusively been targeting video gaming related websites? To borrow one of your arguments: gaming as a hobby is very intertwined with the internet, so if you want to target someone and get a huge reaction out of them (since those guys are probably “doing it for the lulz”), gamers are the easiest and best target.
Rihanna has millions of dollars, bodyguards, and is actually famous. Sarkeesian is some lady with a youtube channel. That is the difference. Proportionality is the difference. Yeah, harassment exists all over the place, but it’s here and now where it’s being targeted at non famous social activists by the gaming population. We aren’t talking about people with platinum albums.
Some of the people I mentioned up there are soap actors, so they are probably less famous than Anita. Since you are talking about proportionality a lot it should be obvious to you that while Anita is not mainstream famous she is definitely famous in the gaming subspace. Her videos have more than 13 million views, and she gets a lot of coverage on all major video game sites.
But anyway, what you are writing isn’t addressing my original point: that there are plenty of psychopaths outside the gaming space up to the point that harassment of actors, musicians and other stars is a daily occurrence. Your generalised statement about there being more psychopaths in the gaming culture compared to other art form-related subcultures remains grossly offensive.