Likely the RC answer is bcz Rome said so. For Rome has presumed to infallibly declare she is and will be perpetually infallible whenever she speaks in accordance with her infallibly defined (scope and subject-based) formula, which renders her declaration that she is infallible, to be infallible, as well as all else she accordingly declares.
How dare you deny Roman autocracy!
How dare you deny Roman autocracy!
:D
Pegged it...again!
Hoss