Why do martial artist focus on specific targets, i.e. the trunk of the body and the head?

Targeting in may systems tends toward the trunk of the body followed closely by the head and I always felt this was troublesome. When I strike, punch, kick, etc. I tend to target whatever is in my path, i.e. the hands, forearms, biceps, etc. or the ankles, side of shins, legs in general and so on.

I believe we focus on the two main area's because we perceive those as the targets that will one, get us the tournament point or two, disable the attacker. Not always true especially if the blow, strike, etc. is direct. The body is armored or protected from direct impacts. One who is well developed can absorb strong blows or strikes directly applied to the trunk of the body. If one takes the counter attack off center line and comes in at an oblique angle along with a strike or blow that is angled in lieu of direct the body loses its natural armor significantly.

Example, I sparred with a fellow practitioner of equal ability and skill. It was a blow that came in a downward angel into the floating ribs with what seemed little or no power that dropped him to the floor. Interesting ....

Is it possible that we have point tournament targeting mixed up as combat targeting? If I can attack the legs and/or arms immobilizing them, making them ineffective and useless does that give me some advantages to gain safety with minimal damage? Something to consider, contemplate and study .... maybe.