They do drop the limits over time. The reasoning is they want everyone to have very structurally sound bikes for safety. You _could_ build an ultra light steel

They do drop the limits over time. The reasoning is they want everyone to have very structurally sound bikes for safety. You _could_ build an ultra light steel bike but it might fail and cause serious injury. With a minimum limit, no one feels the need to make their bikes less than perfectly strong. And when every OEM is making frames at perfect strength under the limit, they drop it.