You can only test a very, very limited number of designs, out of all the pretty much infinitely many possible ones. Someone still has to come up with the designs to test.
And it is very unlikely that you can go from one design to a better one in incremental, A/B-tested steps. It is just as in language: you can test two different novels, for example, but pretty much all the "intermediate novels" you can think of just don't make any sense.
