Replacing $1 Bills with $1 Coins Could Save America Billions

Might be the fact that it doesn't seem like a lot. A paper bill gives the impression that it's a lot of money, whereas a coin just seems like loose change.

My belief is that there's a social contract to not analyse the amount of a tip given in cash at the time it is given. You don't usually count out a tip or say how much you're giving - you're more likely to "slip" them a twenty or the like.

With coins, there's no way to maintain an ambiguity over the amount. If it's a coin, it's clearly a small amount and since coins are very easily identifiable (compared to notes), that social expectation is broken.