Since no one ever checks signatures properly, stolen cards can easily be used for fraud in the US, without needing to shoulder surf for a PIN first.
Some notes on this... Merchant agreements PROHIBIT merchants from asking for ID and DO NOT REQUIRE that merchants check signatures. In fact Visa et al actually essentially PUNISH vendors who do. Famously, Wal-Mart used to have a policy to check signatures and VISA successfully argued that they should not be on the hook to cover fraudulent purchases that Wal-Mart should have caught via signature checks (ie, they said Wal-Mart's employees were inconsistent). So over 10 years ago Wal-Mart changed their corporate policy and cashiers are instructed to NOT check signatures. The same amount of fraud happens, but VISA et al are now on the hook and can't blame Wal-Mart employees.
In Europe, the card vendors were forced by law into Chip+Pin. VISA has more profit that the GDP of many countries and they don't even loan out money. They don't care about a little fraud. Their concern in the USA was users might periodically forget their PINs and pay with cash instead. So they lobbied to keep signatures, and of course our congress persons don't listen to security experts if corporate interests disagree.