The problem only appears when you see mock PMP questions on the topic you just read, and then you realise that most PMP questions are situational. The PMBOK guide does not prepare you for such kind of questions. It just gives you the theory, facts behind each process. But how they fit with each other, how the overall process works, and how to apply the information to PMP questions is completely missing.
Secondly, the questions are pretty tricky, and you have to read them very carefully, to spot qualifiers. See my seperate post on Keywords that change the meaning of PMP questions. Sometimes, people wonder if PMI is really testing on their English reading and comprehension skills, because you can get the question wrong if you miss a single word.
The PMP exam is designed to test real project management knowledge, skills and experience in handling real life projects, as well as your understanding of the complete project management methodology.
Therefore, it is NOT sufficient to just mug the PMBOK guide, pmp exam courseor only to have project management experience.
And in terms of Project management experience, not everyone has experience in handling each and every aspect of project management – some project managers have very little experience on hand to do Risk Management, or Quality Management, or Communication Management, and then there are others who have never done Procurement management, as they do most of the work in-house. So such project managers have blind spots in their knowledge… even though they think they have successfully delivered projects, they lack in certain areas, causing them to do badly in the PMP exam, and many, sadly, fail…