Personally I prefer devs not try to come up with an in-universe explanation of a game rule.
Is that ever explained in that way anyway? We know that the PC wears some super fancy stealth tech, but nothing more. It doesn't make you invisible in the distance (after all, there are scenes in the games where enemies can spot you over very long distance: Sniper-part after the big radio tower in MGS1 [incl. Sniper Wolf scene]), it's just some high-tech body protection (at least in MGS1, 2 and TTS, didn't played any other).