Many Mac security software products DO lock the hard disk. My experiments suggest that the products modify the System file or boot blocks so that the hard disk is flagged as locked every time that the Mac boots. A security INIT is then loaded which requests a password to unlock or decrypt the hard disk -- this is always at the start of the extension loading process, so that INITs that require a writable disk (eg AppleTalk Chooser extension) can function.
Note that the disk may not protected after it has been unlocked if the OS crashes (or you flip the power switch). Depending on software sophistication, the disk may be mountable in another Mac.
Lock/Unlock utilities for hard disks were common in the 1990s, because they allowed content for CD-ROMs to be developed and tested on hard disks.