For many colo centers, there are both weight and power limitations on a per-rack (or per-cabinet / per-row) basis. One wouldn't want a ton of disks crashing th

For many colo centers, there are both weight and power limitations on a per-rack (or per-cabinet / per-row) basis. One wouldn't want a ton of disks crashing through the (often raised) floor! For compute clusters, power is often the limiting factor; for storage clusters, weight is often the limiting factor. If you put these clusters in a general-purpose colocation facility, you'll end up with half-full racks. Density on a per-rack-unit basis is not necessarily important for most of us. Of course, if you're Amazon's size, you build your own facilities, and you can make sure to support the weight and power requirements of whatever custom hardware you can dream up!