Why can't I smell things that aren't there?
Couple of contributing theories:


The olfactory system does not project to the thalamus, which serves as a relay to other higher processing areas.
Lack of a specialized working memory subsystem; we have fairly robust subsystems for movement,visual and auditory buffers of information, what is normally called working memory, things we can hold on for a little while, but it seems they are not as developed or engage the same areas for smell. see for instance:Working Memory in Another Dimension
It is not as useful,used or adaptive ( beyond recognizing odors) as the other senses, there is a relationship in between how much we use something ( could also be the above working memory systems )and the cortical real state devoted to it's processing, this is a competitive part of the brain and in comparison to other senses like vision the portion of the cortex devoted to olfaction is small.  
Another way of looking at it, is that we do have the capability to imagine smells and gustation, but they are not as developed as the others since we don't use them as much.
Anectodally, I would argue that people whose jobs revolve around combining nice smells or tasty things do develop certain abilities like those in vision as well as for those who have been exposed to the same odor repeteadly and can imagine it at will.