Because they tend to last longer and have a warranty, unlike many of the $20 or less ones, plus they have a resale value. Honestly, that is my only reason for buying expensive stuff like that. I have two really like $100 bags. One for just regular errands, another as my school bag. They'll probably last me another 5 years because they're made of quality and durable material. My dad had a wallet made of authentic leather that he bought in the early 90's last him maybe like 10 years.
It's better to spend that kind of money to keep your money in it than to spend a measly $5 on something you're going to have to replace every few months or every year and waste your time emptying and putting things back in order again. Buy cheap food, cheap paper, cheap ink, etc, etc, but don't buy cheap kitchen appliances, furniture, or wallets. You shouldn't have to think about most of those things in the first place.