The reason why schoolbook knowledge fails to come to mind when it's needed is that it is not well indexed in memory. When students learn, they are often not encouraged to try out the new knowledge on problems they face or relate that knowledge to what they already know. So the schoolbook learning forms isolated islands of structures in their memories. They know how to apply these islands to the schoolbook problems they face because that is the context in which they learned it. But the knowledge does not come to mind when they are faced with a problem in a different context.
Memorization has another problem.
It's just not fun.