Do hardware wallets store the wallet.dat redundantly inside? If not, are there such hardware wallets?
I hope I'm missing something, and that somebody can enlighten me.

As I expect you know, information-security can be described as a combination of confidentiality, integrity and availability.
You seem to be concentrating on availability, I believe the makers of hardware wallets are prioritising confidentiality.
If you have a hardware wallet, you are likely to want to also store copies of your recovery-phrase (AKA seed-phrase, backup-phrase) in other secure places e.g. stamped onto corrosion resistant metal plates in safes and/or safety-deposit boxes perhaps in multiple locations. That way, you don't lose availability if the hardware wallet is damaged, destroyed, lost or stolen.
If you keep your wallet.dat on encrypted flash drives, your confidential data is much more exposed than a hardware wallet. At some point the encryption layer decrypts the wallet.dat so that your wallet software can read and write to it. Your data is transferred into a potentially compromised environment with a comparatively large attack-surface. At that point it is probably accessible to malware or vulnerable to a maliciously altered wallet application. This is not the case with a hardware wallet - the secret keys are never accessible to any PC, they never leave the device.