It has, but something fundamental to our atmosphere (even before the great oxygen holocaust) is that it has lots of potential for 'exchange'; ie, its not fully reduced. Part of our earth system depends on the reduction side (us, heterotrophs, etc..) and part depends on things being available to oxidize (co2, previously in greater quantities n2). The fact that both are observable and present would indicate that our atmosphere is being maintained at a stable(ish) state far from equilibrium.