It seems like you and others mostly answered your own question, but another aspect is that often paying with $100 bills raises an eyebrow and comes under more scrutiny - after all, the shop is out $100 if they accept a bad one. People are much more likely to carry and pay with $20 bills, so it’s significantly (I would guess more than 5x) easier to pass a few bad ones off here and there. I think gas stations used to be a common way to work counterfeit bills into circulation, as are retail shops, both of which are going to be less diligent than banks.