Board Thread:Questions and Answers/@comment-25276265-20140807122209/@comment-4913740-20140812122553

I suppose because weapons like this would have the potential to fall into enemy hands. By manifesting REX design specs and exercise data into a tangible, physical weapon, it makes that weapon much easier to duplicate and copy, especially if it were to be hijacked (as we see the Gurlukovich mercenaries attempt to - and they would've succeeded if not for Ocelot).

Apart from that, it's more fuel to the fire. If the US military is making their own Metal Gear's regularly, why can't the rest of the world? That sort of thing could start wars, and while exposing that could also start problems, it could force the US to drop the project permanently.