well because when you have that much money it's really hard to spend it, and big boats are expensive. sports cars are relatively cheap and collecting them is for your star actors and low tier CEOs. islands are too expensive (i don't think they sell greek islands anymore) so only pedophiles that give "tax advice" can afford them. yachts are the right amount of expensive, between 100 mil to a couple of billions. at that range you can comfortably afford them, but they are not small enough to just be a drop in the bucket.

plus big boat cool. you can travel around the world and have the chance to pay filipino sailors fuck all for all their trouble. did you hear about the submarine garage? so awesome.