Surely there's capacity to double or even triple the number of storage bins in existing warehouses by expanding vertically. Humans are limited to 6 ft but robots have no vertical limit. In the future, warehouses could be as tall as skyscrapers with full vertical pickability.
You could build a taller warehouse, but there's a standard height and there are, I believe, often building code things to prevent taller towers like that. And again, if it was reasonable to do it already, they'd have gone taller already.
For the limited subset of a parking garage, which generally has much lower ceilings than is normal for other uses, the cost of one underground parking space is around $30,000.
Plus the reason most warehouses don't build up is building up is expensive. Most warehouses are in cheap land areas so building out is more cost effective.