The main point of accepting returns is to reduce the cognitive overhead and friction of a potential customer's purchasing decision and make them more likely to spend money with you right now. If a retailer doesn't accept returns then potential customers are more likely to shop around for price or selection and become less likely to buy from you at all, and if they do buy from you, it will likely be later, thus costing them some cash flow. Accepting returns makes people buy more stuff and buy it earlier than they would otherwise. And they are more likely to buy it from you specifically.

The primary problem in retail generally isn't revenue or margin, it is cash flow. Creating additional customers or even encouraging customers to purchase sooner than they otherwise would have is a huge win.