Because if not, people will game the system and submit the same paper to dozens of journals at the same time, and journals are for publishing new, original work. And because in a class, you are being evaluated based on your ability to produce new, original work.

Do you also get angry at a track and field meet, when you show up with a motorbike and get kicked out for violating the rules?

But I don't expect someone who writes that much of an aggressively pompous yet uninformed statement to understand. Chesterton's Fence much?