Why is overhead press so hard?

You Can't Control Your Shoulder Blades

But your shoulder blades depend on a few muscles, including your upper and lower trapezius and the muscles around your upper ribs (aka serratus anterior). If these muscles are weak, they'll fail to rotate your scapula up, making your overhead press a struggle.

Is overhead press better than bench?

The overhead press makes you strong in a more classic, or "functional" sense. Because it's done from a standing position, the overhead press seems more like real life than the bench press because you rarely have the luxury of lying down and pressing an object in the everyday world or during a sporting event.