Because it is "hardcore" and "gangsta" and the main audience for those games that are realistic but aren't really is people who would like to consider themselves "hardcore" and "gangsta".
Sad but true.
But the good thing is, those games aren't realistic at all! If you want a real realistic shooting game, play ArmA II. No Desert Eagles there, combat distances are actually proportionate to real life and it's so realistic the US army uses a modded version of the original ArmA as a training sim.
I myself have fired a Desert Eagle!
Useless piece of shit. The weight is insane, the recoil is insane, the price and the price of bullets is insane, and it's just totally deficient as an actual weapon. There's a reason why all police forces and armies rejected it, it's a terrible gun. The only real reason anyone would buy it is because they have a small penis.