Businesses typically pay invoices within 30 to 90 days. In effect, these accounts payable are interest free loans issued by suppliers.
For small firms to gain access to capital, they need to either take on debt and pay interest, or sell equity. Firms with significant market power can also use accounts payable as a source of working capital. In other words, large firms delay paying their bills with little consequence because smaller firms need their business.
This means small businesses are providing interest free financing to companies like Walmart. This is unlikely to change because paying sooner would hurt Walmart's margins, and thus their stock price.
The side effect of this dynamic is that fast growing small businesses can easily go out of business due to cash flow problems if they can't collect payment from customers in a timely manner.