Look at it this way, if someone is about to go to dental school, they're about to take multiple intensive courses for hours and hours every week in anatomy, physiology, immunology, microbiology, pathology, etc., all of which is what's tested on the CBSE, and then study for those courses to pass their tests. That builds the foundation. Even if a pre-dent is already familiar with some or all of those subjects, they'd have to put in hundreds of hours before starting school just to reach the base level of strength in the subjects they'd need to start answering UWorld questions and actually understanding anything. Their classes are already going to do that for them and even if they had pre-studied that much, they'd still need to study more during those classes to get A's on all their class exams, because what's specifically tested and how is highly specific to the course, not just to the subject.
When people ask this question and everyone scoffs, it's because the asker has no idea how much stuff they don't even know that they don't know. The total amount of time and effort someone would have to put in to get even slightly ready for the CBSE before having taken dental school classes would be way more. Even if your pre-studying cut down how much you need to study for classes later, it wouldn't eliminate it. So it's kind of like asking, should I put (random numbers to illustrate a point) 1000 hours in to pre-studying, then 1000 hours into class studying, then 1000 hours into dedicated CBSE studying? This person would spend 3000 hours studying and I'm almost certain they'd only get to roughly the same place as someone who just did the 1000 hours of class studying then 1000 hours of dedicated, because the 1000 hours of self-guided pre-studying definitely wouldn't even get them to the same point as the 1000 hours of class studying would get them anyway, and they'd still have to spend 1000 hours class-studying.
Why devote significantly more time and resources for roughly the same result? Obviously this is mostly conjecture, but I think it would take a ridiculous amount of extra effort to get to where studying in school will get you anyway. Better to spend that time with friends and family and doing things that will be harder to do after starting dental school.