You smell passively in waking life, I think that’s part of the reason it’s rare in dreams. You focus on what you see and hear all the time, but you only focus on smelling occasionally.
It’s not missing in dreams, like previous posters have said. If I actively try to smell something in a dream, I’m usually successful. I can recall smelling coffee and toxic chemicals in my dreams, I’m sure there are more but those are the first that come to mind.
If you make it your dreaming goal and focus on smelling different things in waking life, I’m sure you would start to smell more things in dreams.