Why would they take the risk of hotloading microcode instead of rolling out to small numbers of machines and rebooting gracefully? Unless they run their datace

Why would they take the risk of hotloading microcode instead of rolling out to small numbers of machines and rebooting gracefully? Unless they run their datacenter at redline all the time, there should be plenty of spare resources to shift load and bring a machine down for maintenance. Right? I really don't understand how the risk of crashing programs or introducing undetectable errors outweighs the effort of rolling the update out gradually. surely they have an automated system that can shut down any number of machines without disrupting the network? Then again I have no idea what a datacenter on Google's scale even looks like. Maybe there's some crucial reason they can't shut down any machines at all ever. Smells bad to me though.