There's no fun in a game wherein there lies no challenge. There's a reason why games like Fallout or Jagged Alliance 2 or Icewind Dale I play over and over and over again while I can scarcely bring myself to even finish Action Game XYZ.
Good, comparing turn based to FPS. Imagine JA2 as an FPS. Think about how often your bullets miss. Think about how much damage enemies take. When played out like chess it's fun. When played out in real time with mouse aiming it isn't.
Which is the biggest reason why AI dies so quickly. It isn't because they're stupid; that's pretty easily fixed by just giving more health. It's because no one wants to shoot something 100 times for it to die, and because no one wants guns that don't fire where they aim.
So in order for you to be properly accurate, and in order for enemies to not take a billion bullets, they have to die quickly.
Go reinstall UT3 and mod the bad guys to take significantly more damage, or mod your guns so that you only have a 45% chance of hitting where you're aiming and tell me how much fun you have.