What's wrong with your walls?

Game mode: All
Problem: Bug
Region: Any

There are so many reports of people glitching through walls, animals who seem to be able to pop their head through and hit you whilst inside, NPC’s who fall through floors, placeables which stick through the walls… the list is endless.

I’m not a game designer or programmer but I’m struggling to understand how/why its so hard to prevent this sort of thing from happening.

Why, when you create a model, which clearly has boundaries which the skin applies to, does this not create an impenetrable boundary. Then when two of these models clash, that they cannot cross these boundaries?

Does each model not have these surfaces as solid layers? Please help me understand why this is such an issue.