Because a USB drive is less secure for this use case. If you stick a USB drive into a compromised computer to make a transaction, a virus can steal the private

Because a USB drive is less secure for this use case. If you stick a USB drive into a compromised computer to make a transaction, a virus can steal the private key. If you stick a Trezor or Ledger into a compromised computer to make a transaction, your private key is still safe.