Because no one would grind anymore and you would only see 5 camels in a row following another camel (bots with multi client). He would have some help from him guild mates to kill the thieves and all camels would also be 5*. When he would finish the run he would make around 1 million and can buy what he wants just in a few trade runs.
The game would be boring, because everyone would have tons of money and everyone would run around in sosun gear, but also simple weapons like sos lvl 32 weapons would cost 500 million instead of 5 million.
The game economics would be f**ked up and no one would play it anymore.
Now "Why to be a thief?"
1. It's fun, very fun.
2. You are very happy when you pop someone, because then you know that you are stronger than that "noob", and it's a very cool felling too.
3. You can pwn hunters.
4. You have the best looking job suit.
5. You can hide your lvl.
6. When you come near a trader he says "Plz don't pop me", "I'm low lvl i need money, don't kill me" and it's very fun to hear that.
7. When you kill a low lvl trader he says noob, a**hole and things like that, you ress him and kill again and that is very fun too.
8. It's just cool, try once to pop someone and you will see what i mean (even tho i'm hunter 4 ever i tried thieving and it's great, but i still like being a hunter more than a thief).