Two main reasons.

1. Article is about AI, so I have given the example for an AI datacenter.

2. In pure CPU datacenters, the power dynamics do not change much. I can add more servers to a single rack, but the rack power is again in the 30kW to 50kW range, so you're planning and building for the same power capacity.

> You can stuff GPU servers into existing buildings-

Yes.

> but even with significant upgrades you end up with a lot of empty space on the floor that can't be used.

Yes & No. It's not impossible to convert an old datacenter to support ~35KW/rack capacity, but it's not cheap, and you'll have more worries than holes, piping, building and power. Namely, can your floor handle that much weight to begin with?