It's an abundant resource curse; land is the resource that is wasted.

https://en.wikipedia.org/wiki/Resource_curse

So the question, then, is why build up at all. I'm guessing since this is the NSA lowering surface area for security was a factor.

My office complex is nearly 1 km from one end to the other and a whopping three stories tall, and the third floor is much smaller than the bottom two. If you count the parking lots, it's almost twice as big.