How can there be a shortage in nitrogen in living things when more than 70% of the atmosphere is made of nitrogen?

By volume, dry air contains 78.09% nitrogen, 20.95%oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.
Despite most of the atmosphere being created by nitrogen gas, there's a shortage of nitrogen in soils as nitrogen within the atmosphere has restricted accessibility.
It is, therefore, transformed by the process of Nitrogen fixation, by which N₂ in the Earth's atmosphere is converted into NH₃ gas or other molecules available to living organisms.