People overstate that problem. They say "businesses" as if all businesses are the same.

Bank of America has 3600 branches in the US. At $20/day, that's $72k/day, or $26mil a year.

Bank of America has $3.5 trillion dollars in assets. If they're making 1% interest on that (they're making far more than that, I promise), they're making $35 billion dollars a year. So that $26mil makes up less than a tenth of a percent of their total revenue.

Compare that to the Walmart example. First, at Walmart people aren't just buying one thing. If my cart has 50 items in it, that's a few cents per item. So we're really talking fifty cents to a dollar per transaction. Second, Walmart has around 40 million customers per day. (About half of the total number of people who even have a Bank of America account, and we know that not even half of account holders are going to a brick and mortar every day.)

So, a savings of 1¢ per item is a difference of, conservatively (assuming an average cart size of 10 items), $4mil per day, or around $1.5 billion per year. Contrast with the around $26 million from Bank of America.

They just aren't comparable examples. At its core, the scale issue that you're outlining only matters if you're getting it on a huge number of relevant things it applies to. There just aren't that many actual cash transactions handled by BoA compared to its overall income numbers. Walmart has a huge number of individual items it sells vs it's overall income.