Maybe because for many, stuttering is more of an internal experience opposed to speech repetitions that you always can hear. Many people that stutter are doing everything they can to not be noticed as stuttering. They use techniques, tricks, and avoidances. So you may not hear people stuttering by repetitions, but you can see people using tricks. I often see people using what could be seen as tricks, such as saying “um” before they speak. Though you never will really know if they stutter or if they just say um a lot. Or you may have to pay attention to certain words that they are unwilling to say, but figuring out this requires paying too close attention to others. Even then you won’t know if they stutter on that word or they just simply never say that word. Probably the best way to find out if the people in your life stutter is by telling them that you stutter, if they don’t already know. Then if they do they may tell you about their own experience stuttering. So to find out who stutters, you may not be able to tell. But by watching people’s behavior you can find people that could potentially be a person who stutters.