Because there's no easy way of telling if they are fake. What if they bought a bunch at the Apple store and are selling them at a lower price to get rid of the inventory?
https://www.amazon.com/Apple-MagSafe-Adapter-MacBook-Display...
Now read this review (verified purchase):
https://www.amazon.com/gp/customer-reviews/RNA9AYHGA5JG0/ref...