Because:
1. While many people have had this problem, the majority have not.
2. Apple has acknowledged that this problem is a defect in materials and will replace the case for free (usually in less than an hour).
3. The majority of people who have had their cases replaced don't have the problem again.
and, primarily because
4. if you want a Mac and a laptop, the Macbook is the best value for a student.
However, if you want to buy a Macbook Pro Apple will not object.
Exactly that. Anyone that has an Aluminum Mac knows that although it doesn't crack, it does dent and scratch like it is it's job.
My MacBook has some cracks in it, and is 1.5 years old. Honestly I don't even notice them, and the thing is still very solid feeling and will continue to hold up and take punishment from me for the next few years. Thus far I have
Dropped it
Had it in the front seat of my car and had to slam on the brakes, it went flying forward into the dash
Thrown it about 2-3 feet into the air, and it then slammed back down onto my desk
Tossed it repeatedly
Knocked it against things
Practically used it as a backstop for thrown projectiles (nice little dent where my friend hit it with his old ipod)
Had it fall off a desk chair, falling from the seat to the floor
It still runs as well as the day I got it, and has exhibited no hardware issues except for a bum sleep light (I kind of miss it, it is like it is breathing) and on occasion my Bluetooth cuts out. That's it, those are my hardware issues after having treated my MacBook like crap. So, don't worry too much, the MacBook is great for college. Much better than the MacBook Pro if for no other reasons than that it is half as much money and won't dent when you look at it sideways.