Because the stores don't pay very much for used games.
Indeed, but you're forgetting that you can bypass used game stores and sell directly with sites such as Amazon, half.com, ebay, etc.
So, for example, when I buy a used game for $30 -- because I never buy brand new games, and I don't care to play online anyways, I can sell it again in a few weeks for approximately the same price, minus the cost of shipping.
The used games stores are bad for the industry.
I disagree. If buying new were the only option I had, I would simply do without. It's the same argument for pirates -- if they had to pay full price, they'd simply do without. This isn't something that I personally lose any sleep over. It's very easy to stop consuming (overpriced) entertainment, and there are many alternatives in the world to occupy one's time with.
Fair disclosure: I'm a software developer in the video games industry. (And I'm supposed to be writing some code right now. :-X)