There's a great reason for this. Simplicity. Aiming through a scope or a red dot sight completely eliminates one-third of the complexity of lining up iron sights.
With metallic sights, you are required to line up the rear sight with the front sight and your target.
With a scope, you simply have to line up your crosshairs (reticle) with your target.
It's much easier to learn to shoot with a scope than iron sights, and since most rifle scopes also magnify, your target appears closer, and therefore easier to see, enabling you to place a more precise shot on your target.
People with less than perfect vision are able to adjust the reticle focus at the eyepiece (ocular) for their particular eyes for a clear, crisp sight picture.
Older eyes often have a difficult, if not impossible time trying to switch their focus from a rear sight to a front sight to a target as required without a scope, and it's frustrating, to say the least. Scopes eliminate this frustration.