I think (pardon me if I'm too broad with my answer) it is because almost all humans just exist and go through the motions of life without really questioning purpose, meaning, or even thinking about all the negative shit that goes on in the world. Another reason could be that (especially for people who made up their mind, have a plan, a method, and a date set) they are just going through with the motions as to not alarm or set off red flags with the people around them. They do not want to tip people off which would ruin their plan and attempt when their time comes. I know that once I made my decision that I will CTB, the last thing I ever want is to get saved and forced to live a life that I don't want to live, therefore, I keep quiet and wait until my time & opportunity arrives.