Here's why: because you will forget most/all of what you learn.
"But I'm smart, I can remember it."
No, you can't. In fact, when you start anatomy in med school, you will forget things by exam time that you learned a few WEEKS earlier. How much do you think you will forget of what you learned a few months earlier?
"But I know myself and I have an especially good memory"
Oh really? Can you remember the quadratic equation without looking it up? No? Me neither. And all of us knew that sucker cold at one point or we wouldn't have passed high school algebra. What about the fundamental theorem of calculus? Yeah, used to know that and a hundred other things cold. Now? Couldn't derive something if my life depended on it (unless I could use the power rule).
The point is that we all (yes, even the intellectually gifted among us) forget things we don't use. It's just how the brain works. The same thing will happen to your study over the summer. You won't study hard enough, you'll waste some of your last remaining free hours studying something you're going to forget anyway because you're not immersed in it yet. You aren't really USING it yet. You'll piddle away 2-3 hours a day "studying" something but not doing it enough to remember it in any meaningful way.
Will pre-study hurt you? No, of course not. But it won't help you either. At MOST it gets you 3-4 questions right on your FIRST exam, and you will lose more than that sometime in the early spring when you start feeling the burnout and quit caring for a few weeks until the sheer terror of blowing an exam makes you come around in the nick of time.
This is why I have NEVER met a medical student who walked out of their first exams saying, "Man, I wish I had studied over the summer!"
Relax, enjoy your remaining free time. People much stupider than you have done just fine. You'll do fine too. If you're even considering studying during your last remaining summer, you are probably enough of a workaholic that you'll make the adjustment to med school without any problem.