I've noticed that even when a websites front end is coded in HTML 5 as evidenced by the <!DOCTYPE html>, people rarely seem to use the new HTML 5 <section>, <header> and <footer> tags.  Is it because people don't understand that you're supposed to use reusable classes for styling things within a <section> instead of the full path of tag selectors to the element?
Why are ,  and  tags not used much when I see sites that are in HTML 5?
css;html5;markup