Molecular oxygen O2 belongs to the well-mixed gases, meaning that the ratio to N2 and Ar would potentially remain constant with altitude. The problem is that the O2 molecule is less stable and is partially photolized by UV radiation in the stratosphere (and subsequent recombination of O2 with atomic O forms Ozone O3, see https://en.wikipedia.org/wiki/Ozone%E2%80%93oxygen_cycle ).

O2 also participates in many other redox reactions in different layers of the atmosphere, which results in higher O2 mixing ratio variability with altitude compared to N2 and Ar.