A few theories
1. They simply don't know how to or haven't implemented it yet
2. They don't want players cheesing mobs by circling around trees
3. Collision checks are too taxing on the network
Probably a mix of all three. Being that they tried to do it, but it just wound up being a buggy mess. When it comes to software not having a feature is usually better than having a feature that's super buggy