Agreed, how could we not have datacenter capacity for the GPUs when Meta has shown that you can go from a bare field to an operational datacenter in about 3 months by using tents instead of buildings?
The main problem here is that it reduces efficiency (cooling a large datacenter is more efficient per Watt of dissipated heat than a shipping container) and increases initial cost (building in a shipping container is not actually as easy as doing it in a normal-ish building).
Portability (when offline, you can put a shipping container like this on a truck and cart it around) and availability (no need for a new/refit building, only power is required and could be included in the container with a generator (gas/diesel)) are the main reasons for accepting a higher TCO here.