why do you think when people tell someone not to tell anybody , that they always tell only 1 person?

Weak willpower or poor discipline. It takes effort to mindfully do something for an extended period of time, and for some people they can only go so far before their impulses get the better of them.

But that's most human beings to me. It's like the old saying goes, 3 men can keep a secret if 2 of them are dead

The problem, of course, is if the one person you tell, tells someone else, and then they tell someone and so on, etc…

And everyone knows.

People are horrible secret-keepers. They love to gossip.