Royalty

It's not even the purchase cost; can you imagine the cost of ownership (staffing, maintenance etc). And then the ongoing cost of protecting it - I guess it'll need a navy escort everywhere it goes in addition to its own defences. I somewhat doubt the billionaires will be lining up to pay those, less glamorous, costs.

Historically the royal yachts have been used for training and logistics by the Royal Navy so the use hasn't always been just for royal jollies. I'd have some suspicions that a modern super yacht might not be that suitable for naval training though 😆