If the credit card is swiped with the magnetic strip on the back the merchant gets more information about you. The credit card processor can tell if the credit card was actually at that physical location or not.
I don't think most places check for the signature and it is not only on credit cards. For many years I used to sign my tax returns as Donald Duck and I never got a complain about it.
Nobody has really time for it. Credit card companies do spend a lot of money on chargebacks for whatever reason, but since time is money, no one really emphasizes on verification too much. It's cheaper to calculate some losses from cases where something actually does go wrong.