Not less reliable; more cost effective
It's not that Google literally wants less reliable disks. It wants more cost effective ones. Let cloud software do for disk drives what it's done for servers. The burden of continuous operation lies in the software, not the hardware. When a device fails, there's already a backup copy of the data and a new copy is created on a healthy device immediately after failure. This allows the use of more cost-efficient hardware. It doesn't have to be non-stop, continuous operations for the longest time manufacturable by man. It can have a built in, shorter life without harm to the overall system.