First party games in the gaming world, are games that are funded and published by the platform holder. That being software being made by the same people who make the hardware. While third parties help round out your system's library, first party games drive it. Yet, whenever a recent newcomer enters the industry, they seem to ignore the importance of first party.
I've talked about Stadia before, but still, the fact the Google couldn't get a batch of first party games ready for the platform's launch showed just how poorly planned Stadia's first year was. They waited until the damn thing was launched to start opening in-house studios and signing a developers. Which means we're going to have to wait potentially 2 years before we get anything resembling a first party lineup from them.
Compare this to both Sony and Microsoft. Sony didn't have a history in games when they launched the PlayStation. But they were able to contract a few independent developers to make PlayStation exclusives published by Sony. Within a few years, Sony built their own studios, and acquired a few others. Today, PlayStation is home to some of the best first party content on any platform, that regularly sell consoles en mass. Xbox wouldn't have gotten by without Halo, who's developer Bungie, was bought by Microsoft for the sole purpose of publishing it as an Xbox Exclusive. Halo as a first party game, single handedly justified the Xbox as a platform. Even fucking Oculus gets this with their platform. Hell, I can just say Nintendo and be done with it.
Point is, first party matters. Having third parties isn't enough, if you aren't making games yourself.