When I say to the same degree I'm not saying there aren't people with thousands of games elsewhere. I'm saying there are more of them on Steam and they tend to collect higher amounts of games on average.
Board game enthusiasts don't always have thousands of board games, most console gaming enthusiasts don't have giant walls of games lining their house, etc.
That said, I don't have sources to cite here other than my personal interactions with other steam users on social media and my steam friends list number of games. I could be wrong as this isn't hard data but I'd bet money I'm correct though if there was a way to verify the comparison.
"Jew detected as passenger, locking doors and seeking the nearest cliff to drive off of."