Didn't say it was. But my willingness to pay even the small stated premium does rather give the lie to your statement that said bills are worth only ''face value'', true?
There are a couple of $50s that embarrass even the scarcest $100s in premia, btw. And a 1914 $500 will shock you, what it's worth on the open mkt.
As with anything, the premium for currency notes over face is a direct function of both scarcity AND the number of people who collect said bill(s).
BTW, and no fair Googling, do you just happen to know WHOSE face is on the highest denomination banknote ever issued in the US? Hint: said note was NOT in general circulation, ever.