Because average users use very few apps other than the browser, and maybe an office app, and games.
On desktop media players are largely avoided in favor of Spotify or the like. Traditional instant message desktop software is all but extinct outside of business realm.
Some specialties use more software. Designers use Adobe's stuff. Developers have a lot. Musicians have their own stuff.
But honestly, outside of specialists, gaming is the biggest area, and there you most commonly have Discord, OBS (for streamers), video editing software (for youtubers), the games themselves, and possibly some accessory software for specific specialties (like timer programs for speedrunners). Steam even offers a decent chunk of common paid software gamers might want. The free to download software like Discord often see little point in working with Steam, since the stores offer them little of value. Discoverability is the main thing a store can offer them, but most of them rely on word of mouth instead.
Outside of Games and Development tools/utilities, I honestly can't remember the last piece of desktop software I purchased. Probably Windows, and Office before that. And I have a LOT LOT more software on my PC than the average user.