Why is it important to bargain in South American markets?

Why is considered rude not to haggle with vendors in traditional markets and street stalls?
Is it impolite to try to get a discount on food (market produce and meat)?
What is the cultural importance of haggling?

My Spanish teacher spoke about bargaining in class the other day and I was just wondering why it mattered. Now I live in Trinidad WI and we are not too far away from Venezuela but our markets are completely different. If a vendor has an item and you don’t like the price, you best be moving on to someone who dose. Depending on the time of year market produce will cost more. Some vendors do haggle (the majority don’t) my grandma is the only one I know who gets a discount.