Why the pressure of atmosphere doesn't crush you when you e.g. walk outside? I mean the density of air is $1.26 kg/m^3$, so with $100 km$ above us, it exerts much pressure on you when you walk outside.
If you were a completely empty shell you'd likely be crushed immediately on finding yourself in the earth's atmosphere. But you are filled with stuff (blood, flesh, bones) which is also at approximately atmospheric pressure. If you consider a point on your skin, the pressure of the air on the outside pushing it in is exactly matched by the pressure of the contents of your body pushing it out. So the net force is zero.
To add to Dan's answer - the same thing also applies indoors.
Even with a roof over your head the outside air presses on the windows, doors and walls with the same air pressure, and so compresses the air inside the room to the same pressure.