Why the ozone layer doesn't comes down to the Earth?

The lower layers of the atmosphere including the troposphere, stratosphere, and mesosphere, together are known as the homosphere, because the mixture of gases is so much alike (homogenous). That is, the homosphere is comprised of roughly 78% Nitrogen (N2), 21% Oxygen (O2), and less than 1% Argon (Ar). These gases are fairly well mixed throughout the homosphere. Their proportions remain the same while the density of the mixture of gases becomes thinner with altitude. While it is true that these gases differ in molecular weight, gases do not form into layers in the atmosphere according to weight. Liquids and solids behave differently from gases, so if you were to throw a brick in a swimming pool, the brick would surely sink. However, when a plant releases Oxygen into the atmosphere, that Oxygen does not collect into a layer above the Argon and below the Nitrogen, rather the Oxygen gas disperses and mixes with the other gases of the atmosphere. The molecular weight of Oxygen atoms is about 16 g/mole, Nitrogen is about 14 g/mole, and Argon is about 40 g/mole. The weight of dry air is about 78%(2*14)+21%*(2*16)+1%(40) = 29 g/mole. The weight of Ozone molecules is about 3*16 = 48 g/mole.

Ozone is most prevalent in the stratosphere due to its formation. Energy from ultraviolet light in the upper stratosphere, emitted from the sun, can split an oxygen molecule (O2) into two oxygen atoms (2O). A little lower in the stratosphere, where there are fewer ultraviolet rays, the oxygen atoms (O) can combine with other oxygen molecules (O2) to form ozone (O3). Oxygen atoms may also form back into oxygen molecules again, and ozone may split and form again as either ozone or oxygen molecules. Ozone has a property of blocking ultraviolet light, so as the ozone builds up in the stratosphere, that layer reduces the possibility of ozone forming below it. The thickest layer of ozone occurs at about 32 km (20 mi) above the Earth’s surface; these concentrations are around 15 parts per million, or 0.0015% compared to 21% oxygen. Yet without this thin layer of ozone, and its ultraviolet light blocking properties, life would not have been possible on Earth.