Doorframes are not the only things you can hit your head with. Gardensheds have lower ceilings and sometimes also some staircases are built so that the ceiling is effectively lower.
And it's gradual. I am 6' and hit my head more than my mother who is 5'7. My brother is 6'2 and complains a bit more about that and my father is 6'3 and has the same problem even more often and in the past when buying a car we could often not buy certain cars due to space problems. With 6'6 probably even entering public transport can be an issue as I even hit my head sometimes when I was especially careless and "jumpy".