[dogma1138]

Even if you can, yous implant a keylogger onto the keyboard, and some malware/implant into the screen you get a full readout of every keystroke and every pixel displayed.

If you are going to prevent physical attacks from adversaries that can circumvent basic protection (e.g. FDE) you have to make sure that every device is as secure because the system is as secure as its weakest link.

If your adversaries are just the random person that might steal your PC then any full disk encryption even a cryptographically insecure one would be sufficient because the people who end up dealing with these devices won't have the knowhow or the resources to attack even bad encryption.