Which they do, of course. Customers will try to return things still clearly bearing the label from a different store. They'll ask for a refund on a package of food they've eaten all of.
Still, it's easier to put up a minimum wage employee with zero discretionary power to just give them a refund on practically everything. They lose some money to fraud, but it means that many customers feel safe buying things. Often, they won't return it even if they were entitled to, and may well return to the store to buy the same thing in a size that fits. Fighting with them about it doesn't save enough to be worth the hassle.
Yeah it's been a pretty normal option for the last 50 years or so.