Part of the problem is humans are all slightly different. Thus doctors need to understand the system because there is always something in the details that is unique and so the checklist needs to be deviated from. Once you are in the habit of this is different you quit using the checklist...

Checklists are very useful anyway because they let you focus on and remember the parts that are actually different. However when someone is highly trained it is easy to think you already know everything and not use the checklist.