There are a few reasons:
*Inconsistent pricing: You may have noticed on their website that some games are cheaper to buy new than used (like Marvel Ultimate Alliance 2)
*No disc-repair: There's a game shop I use called Play N Trade which will resurface discs for you for a small fee. This basically repairs CDs, DVDs and, games. At Gamestop they don't offer this kind of service. I think other non-game places do this or rather, back when there were places like Tower Records and Suncoast...Also the sleeve-cases they use for used/gutted discs seem to scratch up game discs.
*Poor selection/nothing older than last gen: A nit-pick honestly since a gamestop's selection is always different depending on where you are. I realize the market for SNES games is lower than that for 360 games but I know there are people out there who collect for their older consoles.
*Selling open/used games as new: When you work at gamestop you can 'rent' games. This is only meant to happen with used games but new games get rented out among employees too. Also, despite their policy on gutting new games, most gamestops I visit almost never have new games that are also sealed in the original plastic.
*No training program: When I worked there, I was thrown behind the counter on day one with no prior training.
*Overall shitty work conditions: If you're a manager you've got it made. If you're not, then you don't. As a basic team member (game associate?) you are basically being pressured by the higher-ups to sell pre-orders and, magazine subscriptions. You're penalized if you fail to make those sales but if you do make good numbers you don't make any sort of commission or really advance in any way. There have been days working in Gamestop when I had to man the register, bundle traded-in consoles and, look at more trade-in items while the manager and her assistant sat in the back of the store (not back room) and played with this new 3DS thing everyone was talking about. Not everyone is going to be like that obviously but it seems like the managers abuse their position in Gamestop based on what I've heard from friends who also have experience working in Gamestop.
*They owe me money (shit work conditions cont.): At gamestop you either get direct deposit (not an option for me when I worked there) or a Comdata card. If you don't know what a Comdata card it's because nobody accepts them (not even McDonalds) and the only way to get money off of them is to use an ATM. Other than the $2 - $4 ATM fee, one can only take out money in $20 increments from ATM machines. In ATM fees alone, Gamestop owes me about $40 but on top of that is about $18 still on my comdata card that I haven't been able to access since I left the company early in 2011. It's a relatively small amount and I really don't think its worth a court battle honestly but it's still not right to offer those two options alone and not one for a proper paycheck. My hours were shit (another common complaint for non-manager, store employees) but a check would have still been nice.
While working at Gamestop, I was told by my manager that I could be replaced within the week. It was the first and only time a manager or, higher up had said anything like this to me at any job I've worked.
If you have about an hour, I think this still holds up nicely (even if it does end abruptly). It goes into greater detail on some of the points I made. Not made by me but something I stumbled upon soon after leaving Gamestop.