I used to teach a Japanese martial art which involves a lot of sitting in seiza [1] and tatehiza. Basically kneeling and advanced kneeling. It was mind boggling how many adults were unable to do it. And I don't mean kneeling for half an hour. I mean just kneeling, period. So many people can't do it. Weak legs and inflexible ankles seemed to be the main culprits. They improved with practice, so it's not like there's some deeply fundamental anatomical flaw at work. Many people just spend way too much time sitting in chairs, to the detriment of everything else.
Someone once told me: "the most ergonomic position, is the next position".