Somehow you’ve gotten your dream home. The cabin in the mountains, penthouse downtown, castle nestled deep in the woods, nuclear bunker in the hills, or whatever. Whats the bit you’d add that might raise a brow from others? The fireman’s pole? The slide? The moat? The hidden room behind a bookshelf where you keep your stuffed animals?
Several factors, depending on geology.
Cost of excavation and supporting the sides from extra earth pushing in.
Bedrock not being deep enough.
Running into the water table, or risk of flooding.
Cheaper to at another floor on top, which adds more desirable floor space.
Tall commercial buildings with sub-basements already have much deeper foundations than that. And their land is much more expensive.