> If sitting on the money has better returns than running a supermarket, why run a supermarket?

First of all, because not everyone starts with inherited wealth. Also because ideally running a supermarket should give you more money even in a deflationary world. Worst thing is that you gain less money on day N+100 vs day N, but it does not mean you lose money or stop gaining it.

Surely most of the people putting their money under the mattress would still need to use a little of that money to buy food.

Yep, exactly why Bitcoin people dont understand their own system.