This is logical conjecture rather than sound scientific knowledge - i imagine in our primeval days, when smell was a far more important sense than it is now - odour detectors were required to alert us to danger, food, friends, and so on, so a built-in immunity to our own odours left our sense of smell free for more important signals, and this has lasted through evolution. Some people must be similarly unable to detect their own body odour, which remains freely available to the rest of us!