why are characters in movies so bad at using guns?

Because it's not an instructional video, it's a story, and the characters do whatever's convenient for the plot to get the point across.

C'mon, that's not true. The main character is always good at aiming.

I hear ya... Seems they only hit their target when it's plot convenient; usually while shouting "I'm gettin' too old for this!" lol...

all you have to do is aim ?

Do not forget about take cover / duck from incoming bullets.