I think the labor has also required less skill over time or at least compared to other companies. Google has very uniform custom hardware. Usually they only produce a few configurations of a particular platform at any time, and there may only be 1 or 2 platforms in a building. They build the racks in assembly lines before they are shipped to datacenters. Software detects problems with hardware, often diagnoses the issue automatically, and schedules it for service.

I think they even used to have a patched kernel that would disable faulty RAM so that they wouldn't have to replace single DIMMs.

I think it follows a common pattern of replacing medium skilled labor with low skilled laborers augmented with very few high skilled laborers.

Also the biggest DCs are in low cost-of-living/cost-of-labor areas. E.g. South Carolina, Iowa, and Oklahoma have the federal minimum wage ($7.25).