Hi,
basically, the problem is the following, to support everything, it would mean thousands and thousands of actions and that would really start to affect compilation time, so it's actually better to have external resources and only bring in what you really need for a particular project.
Each project has it's own needs, not everyone is using the Unity UI, or mecanim, or pathfinding, etc etc. It's a tricky call, I agree.
Bye,
Jean