Because these were bottom of the barrel "budget" guns made as cheaply as possible for sales by hardware and grocery stores to kids and farmers needing a basic inexpensive gun.
They were made of uncertain steels and heat treating, which was good enough for the shells of the day, but not todays modern ammo.
While it "may" be safe to shoot, life is too short and newer, stronger guns are cheap.
In other words, why risk your (or a bystanders) life or eyesight just to shoot an obsolete old shotgun.
As an older gunsmith once said to me, "Kid, we're all going to Hell in a hand cart. Why would you want to grease the wheels".