system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do characters in games have a separated "graphic model" and a "hit box model"? Why can't the graphic model be used to detect hits?
gpt
There's probably multiple reasons, to start though hit detection can be expensive depending on the complexity of the model and the speed of the game (the faster things move the more often you have to check and the less time you have to complete that check).  

The more complex the "hit detection" model, the more intensive it is to check. Take two spheres for example. If you know the center point and radius of both spheres, all you need to do is ask "Is the distance from center A to center B equal to or smaller than the radius of A + B?" If the answer is yes, the objects have collided. This gets slightly more complex with rectangles, but it's still pretty easy.  

Now if you change one object to a polygon with 800 sides/polys (a very simply character model in modern times). You now have to check the position of all 800 sides to determine if it collided with another object because you don't have a single "center" for all polygons and can't simply check distance from a non-existent center.