Why Does The Design Of Yachts Never Change?

Because it works.

And there's a huge variety of designs.

Hmm, I think they have changed too much and was better in the past... But of course the design can always be improved.

I disagree that yacht design hasn't changed. Boats and yachts have gotten much fatter and sort of bloated over the past 30 or 40 years. Powerboats in particular have widened from a typical length to beam ration of 4:1 to 3:1 or even 2.7:1 for small boats. Sailboats are also much beamier and sailboat sterns are much, much fatter and flatter than older sailboats from the 1950's, 1960's and 1970's. Also, the separated fin/keel for sailboats is a reasonably new design change -- almost all sailboats years ago had huge, attached keel/rudder assemblies and much more wetted surface than newer sailboats.

Also, I now see many multi-hulls going by my place on the ICW, both sailboats and powerboats. That is a huge design change from years ago -- there were virtually no catamaran powerboats or sailboats before about 1980.

Another change is the advent of hull windows and larger windows in general. Look at old powerboats and sailboats with with tiny portlights or non at all, and compare that to today's Azimuts and other boats with large hull windows and odd-shaped windows all over. Newer boats are far, far less dark inside than older boats.

Yes, boats still have a pointy end in the bow and a not-so pointy end in the stern, but other than that I'd say newer materials such as fiberglass that can be shaped in curvy, intricate ways, newer sailboat designs that permit much higher rigs and flatter, wider sections, newer engines that are far smaller and lighter than allow more powerboat living space, and new trends in general such as more hull and cabin windows have resulted in quite different yacht designs from 30 or 40 years ago.