Simply put, because we enjoy dueling, so we'll hop on board any opportunity we see for a duel.Why do ENTPs perceive any form of disagreement as a challenge to an epic duel?
Ne/Ti discerns the relative merit of ideas by hashing through them and evaluating all the different possible opposing ideas. The easiest and most enjoyable way for an extrovert to discover opposing ideas is through interaction with those who hold opposing viewpoint(s).
Also, tertiary Fe, especially when it's weak and immature, causes an ENTP to desire for others to view him/her as smart (self-esteem as a function of external validation, ftw). This, coupled with Ne/Ti, says: "What better way to be perceived as smart than to convince and argue with others that my idea is right and there's wrong?"