Most of the people I know that play video games play a wide range, and I highly doubt that steam could have been a fraction as successful as it is if “95% of the people play one game exclusively”.
Although this is a separate issue to the greater point that most people play one game. My bit of anecdata is that me and all of my friends might try out some flavor of the month game or something that caught our eye on sale, but those tend to be one-and-done playthroughs. However, each of us has a game that they play regularly that has an order of magnitude more hours on it than everything else.
I hear your doubts. To be clear: I never said 95% of the people play one game exclusively. I said 95% play two or fewer games. What I should have said was probably 90% of the people play two or fewer games in a year which I think is more accurate.
How can Steam make so much money? Well playing two games doesn't mean you own two games. Many many games are "purchased" on Steam and never played at all. The two games do change over time, though not quickly. Also, there are a lot of people out there with Steam accounts!
Besides my kids as evidence, I can't really link to research I look at. I am a 25 year game industry veteran who runs the interactive division of a small entertainment company. So I guess I can site myself as an expert :) Which you can reject.