Why is it dangerous for human being to touch one of contacts in AC wall outlet?
This is a fallacious question, meaning bad logic from false assumptions.


  is it dangerous for human being to touch one of contacts in AC wall outlet?


What is dangerous to be ignorant (unaware) of the reason how it "could" be dangerous.

If you touch AC line AND also have ANY stray or direct contact with Neutral or Earthing ground then current may flow.

Earth leakage can be a humid weather wearing damp socks on a dusty floor which may be uncomforatble leakage but not lethal.  Or you may have leakage resistance or stray return current path "conductance" or "dielectric impedance" to conduct to the earth return path, which is also shared with the NEUTRAL wire. 

The  "ground" may appear on the surface of an appliance or plumbing or peripheral PC case that is also "earth" bonded or  grounded meaning 0V connected to earth bond wire.

Voltage just has the "potential" to be dangerous but it is current that you feel or contracts muscles or becomes lethal. This depends on V/R or the leakage resistance and insulation breakdown voltage that has the risk to conduct current that is be dangerous above a certain threshold.