Either you buy a decent gaming PC and use it personally a few hours a week, or Google buys a decent gaming PC and uses it ~50 hours per week across ~10 users on average.
Google can probably also utilize that gaming PC during the other 118 hours of the week with batch jobs, indexing, etc. Your PC will just be idle and depreciating at 3am.
Google's costs are clearly lower.
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.
The reason is that game devs are able to optimize their games specifically for the GPUs in the console hardware. Google is presumably betting that if they get enough market share, devs will start to optimize for _their_ hardware, and reap similar rewards.