I mean it's not like it takes any more time.

If there wasn't mandatory homework/ or lectures I'd agree. However if you study before you might completely understand the material(because you spent 2 hours going through the book already), but you still have to do a 2 hour problem set.

I honestly find the difference between the top and lower students has little to do with work ethic, but instead the ability to accurately judge what they do/don't know. A lot of the lower performing students find one pattern when studying and assume it is true everywhere, which by extension meaning they are done studying that type of problem despite having no actually understanding.

So the real benefit to these people prestudying in my opinion isn't having the professor to ask questions. It is having the professor poke holes in the "pattern" they think they found, so they can readjusted it before the exam. But most people who consistently get As(like I assume you probably do), are also capable of poking holes in their own theories so the professor is as necessary.

Don't get me wrong if the professor/material is hard enough it is always nice to have a second person to poke holes in your theory, and the professor is a good choice. But I find a smart group of friends has the same effect. So I don't necessarily think prestudying is objectively better or less time consuming for everybody.