@BOING
We can question that logic but to the designers of the system it probably seemed eminently sensible.
Of course the assumption on which that logic was totally based, now shown to be incorrect, is that "hijackers" always start out outside the door. Obvious in retrospect (and not impossible to imagine prospectively) is that mental illness, post-hypnotic suggestion, the deception of a sleeper, and other such non-physical (mental) methods of incursion are impervious to a physical lockout.
That realization is also why the 2-person rule is at best a half-measure of address -- once a mental danger enters the cockpit by any means, all bets are off.