When you plug a hardware wallet into a computer it can't get the keys off the wallet. It can only ask the hardware wallet to sign a transaction, and the hardwar

When you plug a hardware wallet into a computer it can't get the keys off the wallet. It can only ask the hardware wallet to sign a transaction, and the hardware wallet asks the user to confirm. When you plug a thumbdrive with keys on it into a computer the computer can just take the keys. It's the same as the difference between a YubiKey and a thumbdrive with GPG keys on it.