Didn't our parents teach us this? I know it was for a noble cause: Keep us safe and away from people who might do us harm.
But the things we are taught as young children calcify in our minds as unconscious beliefs that direct a lot of our behavior later in life, without us even knowing.
Who hired you for your job?
Who was your last customer?
Who was your last date?
All strangers. At first, at least.
The sum of people you know or have even said hello to thus far make up such a tiny percentage of the population, you'll never make much of a difference if they are the only people you talk to. If you want your name known, or you want to make a lot of sales, or you want a lot of people following you, you better talk to some strangers.