Beacause all gases are soluble in all gases, and all gases are solutes of other gases.
Translation: all gases eventually mix equally in a ‘closed system’. At the molecualr level, Brownian movement does it. At the atmospheric level convection and winds do it.
It took 20 posts for someone to state the correct answer, Brownian motion.
As an aside, CO_2 is often in greater concentration near the surface, but that is because soil and sea water slowly emit CO_2 and they also are sinks for CO_2 varying with season and temperature.
Gravity can act to separate heavier molecules from lighter molecules. So I want to separate Uranium 235 from Uranium 238. Earth’s ordinary gravitational field is far too small to develop a significant concentration differential. Recently super centrifuges generating very high gravitational fields can make measurable differences, and cascades of such can perhaps make enough Uranium 235 for weapons.