How receptive are literary agents to getting media tie-in novel queries? Is there a reason they aren't listed in the genres that the agent will accept, or are tie-ins considered just part of the 'fiction' genre?
To answer this question, you have to understand what a tie-in is: it's a piece of fiction using characters licensed from a rights-holder like a movie studio, a literary estate, a gaming company, etc.
Usually the way a tie-in novel comes about is that the rights-holder will approach publishers with a property or publishers will approach the rights-holder. Several publishers, for instance, sought the rights to do "Monk" novels and Penguin/Putnam eventually won out. Only after the rights are licensed to a publisher do editors seek out authors to write the books. That's when an agent might enter the mix.
So it wouldn't make any sense for you to query a literary agent with an idea for a tie-in novel...or the manuscript itself... unless you are the person who holds the rights to those characters. Otherwise, what you're asking an agent to do is sell your fanfic...and no agent will do that. That's why tie-ins are not among the genres that agents are willing to consider for submissions.
If what you'd like to do is write for an existing line of tie-in novels (like, say, the STAR TREK series), querying an agent isn't the way to go. Agents simply aren't looking for new clients to take to the editors of tie-ins...for one thing, there isn't enough commission money in it to make it worthwhile. If an agent is going to suggest someone for tie-in assignment, it will be one of their current clients.
So, in general, you need to already be on a editor's radar to get an assignment for a tie-in... it's the editors you need to reach, not agents.