I disagree with your statement. 20 years of head start could also be seen as 20 years of polishing a previously non-existent service. Today Steam's features are widely known. Just make an app with same or similar features and you're golden. The blueprint is there!
As an example of "what-could-have-been" I would present Immich which is an alternative to Google Photos and iCloud. Developed by a tiny group of people. It does lack some features that Google Photos/iCloud has. But for the most part it easily could substitute anyone's photo-storing needs.
If bunch of people with no money in their pocket and only free time off work managed to develop a fully functional, well polished photo app that would rival market giants, why cant market giants make something that would rival Steam?