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.
So I'm working for a friend's company at the moment (friend is a small business owner who designs websites and a bit of an entrepreneur) anyway, I've persuaded him that we should research the empirical literature on what makes websites effective (which we've done a lot of now) and to advertise ourselves as being special by reason of doing this (which we're only just starting to do).
One thing that I found absolutely remarkable is how unfilled this space tends to be. Like a lot of things in the broad area of empirical aesthetics it seems like there are a lot of potentially useful results (c.f.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3485842/ ), but they're simply not being applied- either as points of real practice or of marketing differentiation.
A fascinating gap.