Point is, whatever the reason for the obvious security flaw, it makes it so that Mr. Kellog's there getting inside the Vault easily not that hard to believe.
All right, remember how the airborne FEV from west-tek mixed in with radiation caused wastelanders to be poor choice for FEV dipping my the Master? And that's why he needed humans pure of such toxins? Maybe the process of putting someone into a synth follows the same rule-set? And that most of the institute had been exposed to the crap and so their synth versions could never achieve the vision they wanted. So they went in search for a pure human who's brain would not be all mucked up. The younger the better as it'll be easier to brainwash someone who's young to follow your ideology and orders. And once they found the baby? Oh boy, that was the jackpot. Perfect for their project.
/pure speculation.
I think that's exactly what it will be.