Because SALVATION IS BY FAITH (Eph 2:8,9), and NOT as the result of "Works" regardless of the nature of the works. If you mess with THAT TRUTH, then you've GOT NOTHING.
IF a person doesn't enter by the only door: God given FAITH in the perfect SIN OFFERING that Jesus made on the Cross for you - then you don't get in.
It makes no matter how many Religious Ceremonies/Activities/Baptisms/"Hail Marys"/etc. that you happen to do, or how many "Bathtub Virgins" you have in your front yard.
What the Catholic (Roman and otherwise) always gets backwards is that "WORKS" are NOT, never were, and never will be SALVIFIC in any way.
"WORKS" are, however, the result of the intrinsic nature of BIBLICAL FAITH. That's to say that IF you are living in a BIBLICAL FAITH RELATIONSHIP with God, then "Good Works" will happen. they contribute NOTHING to your salvation, and are simply the RESULT of it.
Note: I'll agree that IN SPITE of Catholicism, there are individuals who do find their way to truth, are convicted of their SIN, and do call out to God in FAITH (that He gives) and are Born Again.