Why the composition of atmosphere is so specific and steady with 21% oxygen and 78% nitrogen?

Nobody knows for sure, but you should surely thank your luck for the kind of air mixture you have been provided with by natural processes that took place long in the past.

Consider this: If the abundance of oxygen was any greater, all life would be at risk. The probability of forest fire increases by 70 percent for each 1 percent rise in oxygen concentration above the present level. If it formed 25% of the air instead of 21% now, oxygen would leave all land vegetation in raging conflagration. Everything from the driest Arctic tundra to the wettest tropical rain forest would burst into flame. And if the level of nitrogen, which is largely responsible for air pressure, were to fall to 75% from its present level of 78%, our planet would enter ice-age – possibly permanent.

It seems that the present equilibrium between blow hot and blow cold is nothing less than a miracle.

Additional reading:
Atmosphere of Earth (Wikipedia)

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