Why are there no hydrofoil yachts similar to this?

[karldiesen]

With active hydrofoils it would be more comfortable, prevent seasickness and make yachts go much faster/ longer. They could be retractable too, to sail in shallow waters.

 

[kashurst]

not as sophisticated but a lot cheaper:
http://www.paritetboat.com/en/boats/yachts/hydrofoil_yacht_looker_440s/