I think Superman 1 isn't terribly uncommon, relative to the era. But in grade it's virtually non-existent. And as a consequence it seems like many copies that surface are restored, at least more so than other GA keys.
Rather than throw out ignorant statements I decided to look it up. CGC census says:
Supe 1: 68 universal, 90 restored 57%
Cap 1: 88 universal, 74 restored 45%
Bat 1: 128 universal, 136 restored 51%
Action 1: 42 universal, 28 restored 40%
Marvel 1: 30 universal, 33 restored 52%
Tec 27: 35 universal, 36 restored 51%
So that's numerically true.