Because. And that should be good enough.

But if I over think it I would say because that is the way it has been done. And any change would add $0.01 to the price. That might be enough of a difference to encourage someone else to enter the market. That new person may not have the quality morals that the old timer had.

Sometimes the status quo is a good thing. If there is a safety product that works and is cheap do not ask questions.

Also the paint or ink on the one part available to read would risk becoming an isolator. The normal application would be a rub to fill the stamp. Bad idea. And unlike a quick metal stamp a screen on the glass expensive.