why do mobs get stuck in walls?

Why do mobs get stuck in walls? like, id understand mobs that are made of armor stands, kinda, mainly cause I dont understand how armor stands work but I digress. Why does every mob have a chance to get stuck in or go through a wall? like, even mobs that use a player model get stuck in walls.

 

They are ghosts

 

The teleport/charge spells that mobs cast or the uppercut spells that warriors cast make mobs move faster than normal. The collision model works very well with flat walls, but when you have a corner it doesn't work as well. Let me draw you a picture:

All mobs and players are boxes when dealing with collisions. When something moves fast enough that it's position next tick (fairly small but not minuscule time period) is past the middle of a block, it gets pushed back the opposite direction. It's easier for this to happen diagonally because when a mob is in a block on a wall, it can still get out, while it cannot get out of diagonal blocks:

 

Teleport checks a bunch of positions along a line and once it finds a spot is in fact not air, it goes back one.