Because they wanted to get the cheeese and Shrek
The pros were that you were being protected by the guild in case of something bad happening you like getting hurt or similar. They would help you and your family for example. The cons were that you wouldn't be able to dictate the price of your wares which was done to prevent exploitation of consumers, so even if you were in a position to establish a personal monopoly, you couldn't and you couldn't become richer than what is the adequate wealth for a merchant.
