People's intuition is based on what a rack in a typical modern data centre would be like. i.e. High end server CPUs and multiple GPU's per box, which really should be factored in since they're the bulk of the power draw and heat production. Put a few of these in your basement and you're going to need special cooling and power solutions.

The pictured room is not full of high-performance computing machines, even for the day. (HPC was ruled by mainframes back then, although if this picture was taken well into the obsolescence of the room, high-end workstations (e.g. Silicon Graphics and Sun) may have been gaining on them for some applications.) This room was built for getting a couple hundred callers networked into a BBS where they could access the same files, text chat, and perhaps even MUD a little. This was a fundamentally different beast.