system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do hitboxes exist in video games? Why can't it just be "if object A hits 3D model B, it hits"?
gpt
Brumisator is completely wrong, networking is not why we use hitboxes. As evidenced by hitboxes are always used in non-networked games. 

Determining if something hits a 3d model with full accuracy is computationally expensive. To determine if something "hits" a 3d model, you need to draw a line between two points and determine if it runs into any of its polygons. In Crysis 3 the characted models have 60,000 polygons. Imagine having 20 of them on screen. Without hitboxes, that's 1.2m calculations (Simplfying it as 1 per polygon when in reality it's actually a group of 30 or so individual calculations) every "simulation tick". There may be 60 ticks in a second. This gets out of control fast.

Enter the world of the hitboxes. Turns out that for games "good enough" is really good enough. They don't need 100% accuracy cause it actually doesnt matter in all situations. Hotboxes allow developers to define a region of space where if you detect a hit, you hit the character. This is typically a literal box in space, 6 polygons for a region of space to test instead of each polygon. Depending on what they're hitting, it might actually be a circle which is even easier. You do 1 calculation to detect a hit on a spherical region of space.