The bunker was dug, but they had to go through a ton of work to reinforce it, pour a foundation capable of holding server racks without disturbing the existing

The bunker was dug, but they had to go through a ton of work to reinforce it, pour a foundation capable of holding server racks without disturbing the existing structure, waterproof it, and more. Maybe the seemingly advanced building techniques they described in the blog post are also required to build an above-ground datacenter. Or the cost of doing it (and doing it below ground) isn't drastically greater.