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).