Why is Hindu food not allowed? How are they najis?

Why are we not allowed to eat Hindu food ? The answer given is coz they are najis! How are they najis? And during the Prophet's time, there were many non-Muslims, yet they had a good relation with the Prophet. According to the reasons given to me, it does not really make sense that the Prophet would actually say no to food brought in by the non-Muslims. Aren't we just hurting a person's feelings and getting sins and insulting food? I always fail to understand this.