But what are you basing that that off of? There are a ridiculous number of confounding factors that might explain why a particular website doesn't conform to the latest studies in web usability (money, time, the site gets tons of hits already, management is hard to talk to, etc) outside of "professional web designers don't seem to use [empirical web design data]."

And if you go beyond the web designers themselves then you are really just asking why companies/corporations don't tirelessly invest in making the best website possible.