Agreed, though with inflation $100 U.S. ain't what it used to be! Traditions die hard - several attempts at a U.S. $1 coin have encountered huge consumer resistance*, but if someone had suggested a 10¢ or 25¢ bill way back when they had about the same purchasing power as a dollar does today, they would have been laughed out of the mint.
Some of the rarity of the $100 bill probably has to do with credit/debit cards, which are the normal medium for buying anything that would have used a $20 or $50 bill "back in the day" and would suggest using a $100 bill today if anyone was going to pay cash for them. Whatever the reason, store clerks don't get too many.
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*You Canadians, along with most other countries I've been to, have this one right.