But sometimes it does. I worked in audit for several years and the auditors will challenge any estimate made by management that the auditors deem material to t

But sometimes it does. I worked in audit for several years and the auditors will challenge any estimate made by management that the auditors deem material to the financial statement readers. And the useful life of how long computers last is an estimate by management. While at most companies useful lives might not be an estimate worth looking at, Alphabet has enough computers for this to be a material estimate (likely due to GCP) that is subject to management bias. The public auditors are there to ensure (to a reasonable extent) that management can't manage earnings by changing estimates like this.