I'm trying to make a hole open up in a primitive, in this case I hooked the vertices of a hole in a flattened cube. The scale animates so that the inner hole opens after a sphere lands on the cube.
To explain this further, a "Convex Hull" is an approximation of your shape, NOT the actual shape. For example, the object in the back is a convex hull for the torus. Since it's 'convex' it cannot have holes or indentations - that would be concave:
For physics simulations, the simpler the shape, the faster it is to calculate...that's why there are so many simple, primitive options for the Collision Shape dropdown. If you can get away with an approximate of your mesh then it's quicker to simulate. In my torus example above, if you never care if anything goes through the center hole, the convex hull will work fine. Or, another example, if you had a Suzanne bouncing around, you probably could get away with a way simpler convex collision mesh like this:
However, in your case, you want the actual mesh shape to be the collision, so you need to choose "Mesh" for the collision shape for it to work as you expect. This is slower to calculate for the physics simulation because it needs to check collision with every face of your mesh. Yours is simple enough that you won't likely see any performance hit, but with a more complicated scene you might.
In your case, if you do care about performance, I would make something like two cubes with collision on them that slide apart as the hole opens, rather than using Mesh collision.
Hope this explains in a bit more detail for anyone who is interested!