The cost of building large 5+ floor concrete structures that can take significant floor loading in kilograms per square meter? Building vertical anything isn't cheap, this is why you see massive cloud scale datacenters built into "shell" warehouse buildings that are tilt-up concrete walls, the whole thing 1 story high sitting on a concrete slab. Usually in some places where land is cheap in $/square meter. Example: Datacenters in The Dalles, OR.