As people evolved and began to use reason more, they didn't need to smell things quite as well as other animals did. They could use their eyes and their brains in a different way. There is one way your sense of smell is different from all your other senses. After sensing a particular smell for a while, your sense of smell gets tired. When you first come into the house, you can smell dinner cooking, but after that your olfactory nerves get overtired and then you don't smell anything at all.
Some people develop their sense of smell for a special use.
A perfume maker can tell all the different flowers from each other by their different smells. A wine maker has the same talent for telling wines from each other by their smell. Smell is one of the ways we have of knowing about our world and enjoying what it has for us. Close your eyes and smell a rose, or after a long winter, go outside. That nice green smell tells you spring is here.