Encountering people in plate armour in D&D should be far rarer than it is because encountering it in the real world, even at the height of its use and distribution, was not a regular event since most of the time, nobody was wearing the stuff since most of the time people weren't engaging in battle, they were living their lives. But D&D doesn't represent reality. People walking around in plate armour all the time is normal in D&D. Thus, the rules don't represent reality, they represent the D&D world and it's reality.