With the exception of Bioshock Infinite, I can't remember the last AAA game that wasn't $49.99 on PC day of launch. That's 20% below consoles, and they usually drop another $10 within 30-45 days. For price conscious folks, a 30% savings on every game adds up to being able to afford an entire extra AAA game for every 3 that they buy.
The reason for the price discrepancy is because copies sold can't be shared and passed around, so they're willing to take less per copy.
I'm waiting for Sony or MS to wise up and start pricing their digital downloads at or below Gamestop's used price. If they want to break used sales, that's how to do it.