People aren't willing to pay for mobile games any more than for other apps.
It’s curious that you had to specify mobile games. That seems to indicate you understand those are their own class of product (often more slot machine with extra steps than software or game) than what the conversation is about (Steam, thus desktop games).
The App Store—which, by the way, I was thinking of the one on the Mac—was merely an example to represent how companies understand and separate games from other software. I could’ve also made the point of games being seen as entertainment rather than software by pointing out Netflix has movies, TV shows, and games, but not other apps.
Apps (“software”) and games are fundamentally different in the public’s perception. Look at the App Store, it has two different tabs for games and Apple is even making a separate app for them.