Then what you're looking for is a Dell XPS or an Alienware and those are both in Mac Pro price ranges, which is geared more for gaming.
iMac is more run-of-the-mill computer pricing, if a tad on the higher side including a better monitor.
You're expecting a Dodge Viper engine in a Dodge Neon chassis at a Dodge Neon price and that ain't gonna happen. Besides, as other people have said...you can upgrade the graphics chipset in the 24" iMac so if you require more power, yes - you have to pay for it. What's so appaling about that? I don't expect to get a GeForce card in a $400 Dell Dimension (which comes with Intel GMA).
Again, you're comparing apples to oranges (no pun intended). No computer Apple makes is geared for serious gaming, and the closest thing is a Mac Pro, which is in a similar price range as an XPS or Alienware (and technically, the Mac Pro will blow the doors off of those computers).