Because every conversation begins with this look:
Does anyone actually think the Christians who participate in this event want to have a conversation? Not a chance. They want to preach and convert. They have no desire to hear your arguments against their beliefs.
Can someone just photoshop these cards (PDF) and make them Day of Monologue cards? People could print them out. Then, if someone hands you the JesusCard, you have something to give them back.
(Cartoon via Jesus and Mo)