Because when people are willing to pay for brands. It's not always about paying for simply the logo. It's about what comes with the logo i.e. quality of the product that has been acquired through time.

For instance... I'd rather buy a certain brand of cheese/chips/ketchup than the generic store ones because that brand just tastes better. And it's worth the difference in price.

Also... Some people rather pay extra for the logo simply cause they want to be associated with the class status that comes with the product.