Oracle are actually subletting part of Bynet's new-ish Har Hotzvim facility which has 2,400 racks but is probably power-constrained - I believe it had 16 MW when it went live, with passive provision for doubling that. Even if it's since been upgraded, that's still only 13 kW per rack which is pretty stingy these days.

As a very rough rule of thumb building down is about 4x more expensive than building up. So probably worth doing if you're Shin Bet (who I believe also have space in the same dc), but for the likes of Oracle it's only going to be used to serve clients with specific security requirements. Think of it as a halo project - more of a marketing exercise than something that's actually going to be used by the average customer.

The same goes for datacentres hosted in cold war bunkers etc - they always end up being too constrained in one way or another to be useful. The big facilities end up being built above ground and rely on geographic redundancy rather than trying to make themselves (literally) bomb-proof.