For many years now, PC Game Pass has only been accessible on Windows 10 and 11 through the Microsoft Store. However, Steam is the most popular storefront to buy and purchase games, and as the subscription service becomes more popular, it's a noticeable omission.
While it's unclear if Microsoft plans to put PC Game Pass on Steam, Valve head Gabe Newell is willing to work with the company to put PC Game Pass on Steam. It's unclear how this would work at the time, but if both parties worked together, I'm sure a new solution would emerge.
Newell said the following in an interview with PC Gamer.
I don't think an [PC Game Pass equivalent is] something that we think we need to do ourselves, building a subscription service at this time But for their customers, it's clearly a popular option, and we'd be more than happy to work with them to get that on Steam.
In January 2022, Microsoft announced that Xbox and PC Game Pass now number over 25 million subscribers. It'll be interesting to see how the company encourages this growth in the future. Maybe placing the service on Steam would attract more customers on that front.