Valve literally hosts petabytes of game data and allows any user to download them at any time. That’s not nothing, data storage is very expensive, and users are charged nothing for it. They provide a service to not only the customers, but also the developers. Steam has so many backend features that allow devs to skip so many networking steps that can otherwise be a huge nightmare. Not sure why you think they are literally just a webpage that has a purchase button next to a game.