Why the pressure of atmosphere doesn't cruch you when eq. walk outside?

Because we evolved in it. Same reason deep sea fish don't get crushed at much higher pressures, and why an astronaut's hand exposed to a vacuum doubled in volume: http://en.wikipedia.org/wiki/Space_exposure

If our earliest single cell ancestors couldn't survive that pressure then none of their descendants could have either, and each descendant in turn has to be able to survive in those conditions or it's an evolutionary dead end.