You give someone your business card.
They put it in their pocket, intending to follow up with you later.
But that evening, or later that week on laundry day as they're cleaning out old receipts, lint, and other random objects from their pockets, your business card gets tossed in the trash along with everything else.
Or even worse, the person you give it to takes it promising they'll contact you soon, and toss your card in the trash as soon as they walk away.
Was it really worth it for you to spend all that time putting thought and care into designing, ordering, carrying, and then giving your business card to this person?