
Steam, for example, didn't need to fund a Steam Deck - it just happened to be an area worth consideration, over a decade after Steam was released. This allows them to, in turn, better fund hardware development or other console-specific initiatives that they need to survive. Essentially, in return for providing you the convenience of all-in-one integration (All the tools you need are provided to you for community/management/purchasing/safety/etc), you contribute to the costs of maintaining that burden. Heck, most console services help build your game library as a part of their subscription models - even if it is ancient emulation of old games.

which can't be "costed" in the same way from their cut of video game sales - there simply isn't enough volume, and they have too many costs tied exclusively to themselves with regards to hardware costs/staffing/supporting developers with integration, etc. At the same time, Microsoft, Sony, Nintendo, and anyone else who enters the marketplace as a closed-ecosystem, are essentially providing community tools, partial-control of matchmaking (Validation of usernames, ban controls, etc), voice chat servers, hosting of content, etc etc.
BUY PS3 GREED CORP PS4
You aren't allowed to go, for example, "I'll just add X-Box Live profiles to this PS4 game" without extreme amounts of negotiation, because console manufacturers are responsible for their systems ecosystem. When you develop a game for a console manufacturer and add multiplayer, you are essentially tied to their infrastructure. Why does this make such a difference? Well. They can do this because they don't have to also fund a hardware ecosystem, every part of the user experience, etc etc. through the sheer volume of game sales made. Meanwhile, services like Steam essentially fund their community tools, etc. Where server demands are high for a developer, say for an MMO, there is still a cost to the end user however. The only person shouldering a mandatory cost for matchmaking is usually the game developer, and the costs for maintaining their matchmaking servers are accounted for in the cost of their game. GoG is largely reliant on developers sorting themselves out.

Steam does what it needs to do to make it a good storefront.

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None of your PC part manufacturers are providing you gaming services (Under normal circumstances ahem), you aren't obligated to use a gaming service provider like Steam or Origin, and if you want you don't even need to use Windows! Equally, that means no one provider of goods/services has to provide you - or developers - with an excessively extensive suite of tools to help facilitate matchmaking. PC's have no one inherent "ecosystem" everyone is tied to. I'll try to give a simple-but-cover-most-bases explanation as possible.
