The high end PC customer can keep upgrading frequently, while Google will need to upgrade their entire data center to offer the top experience every few months. They will settle for a reasonable good/great experience for their user base, while the High end PC owner will always be above the Stadia standard setup.

So it's not that Google's cost is higher than the high end PC owner, it's just that the high end PC owner is willing to throw more money at it to stay on top compare to what Google is willing to throw at its entire user base.