That's changing as of this month. Most banks have already issued new chip cards, which require a PIN. If a merchant is still using the old swipe and sign system, now they'll be liable for any fraudulent transactions if the customer has a chip card.
I'd like a chip card that requires a PIN (and that is accepted widely in the US) because such a card would make unauthorized charges less likely after the card is lost or stolen, but was not able to find one.