The differences between 2D and 3D physics may, at first glance, not be apparent or obvious. True, one operates in three-dimensional space and the other in two dimensions only. Yet, these aren't the only differences. This becomes quite clear when we look at the features offered by both types of physics engine.
Important to remember is that 2D physics only work on the (pictured) 2D plane, while 3D physics involve the entirety of the world space:
When we look at Box2D, we see that it works with rigid bodies only, sticking to primitives for the body shapes. This is because 2D graphics are generally done with sprites, which do not have a mesh and generally match well with basic shapes.
This is quite unlike 3D physics engines, such as Bullet, PhysX, and Havok. While all of these can work with sprites and limit themselves to a two-dimensional space by ignoring the Z-axis, you would end up using such a small subset of these engines' features that it would not be worth it...