Lol at the downvoters. I'm sure you can build a lifestyle business around a CMS that spits out a static site but honestly, unless your audience is Hacker News, people don't care whether it's static or dynamic and often times they'll ask for that one thing that isn't static and breaks your ability to sell to them.

So yes, this is a feature and not a product as evinced by the numerous plugins and projects that take existing dynamic web application software and can produce static sites (Flask Freeze, Drupal's plugin, I think even WordPress has one).

[EDIT] Which means, what's to stop the existing "site builder" companies from doing the same? They don't sell the feature (static builder from a dynamic site), they sell the product (build yourself a site), and people buy it. The "background benefit" is a slightly faster site and who does that really matter to? Not the customer. The service - reduction of bandwidth and server resources; the could easily introduce the feature, cut resources, and keep the margin.