That's a bit like saying coders can just make a game then license the IP to TV stations, moviemakers, writers and merchandisers as a secondary revenue stream. It happens but it's rare enough that it may as well not exist for most. Musicians on the other hand almost all play gigs (as well as being able to sell their music to videogame makers, TV shows and movies), and the movie industry practically invented merchandising as well as other avenues of income.
That's like saying someone can go out and write books, get them published and sell enough copies to make a decent living doing it. It happens, but it's rare enough that it might as well not exist for most.
"It's hard" doesn't refute the fact that it's possible. It's a lot more possible than increasing sales by targeting piracy.
Literary agents liaise between writers and publishers/producers etc. They have nothing to do with building up a fanbase, most authors do all of their marketbuilding themselves, in their own time, on their own dime.
I have a family member who is preparing to release a book at this very moment. Her literary agent's first action was to put together a marketing team to promote the book in order to grease the wheels in selling the book to a publisher.
A good, modern literary agent also performs (or subcontracts) the duties of a manager and marketer as relying on just liaising with publishers vastly reduces their potential income where things like Amazon and iTunes are open to anyone with or without an agent or publishing deal and where a little buzz can vastly increase the publishing deals (and thus comission) they negotiate.
And it's been pretty well established that there's few (if any) people who pirate media that would run to Amazon or iTunes and buy something if they couldn't obtain it via piracy.
Certainly established to the satisfaction of people who pirate books anyway.
Are you really one of those mindless idiots who jumps on the cock of the media conglomerates and believes that every connection to a torrent swarm is a lost sale? I didn't think there was really anyone who believed that and could actually get a computer to work, much less find their way to /.
Freely released books are a very different matter to piracy, especially from creators who can least afford it.
One of the first books I got for free was a the first piece of Twilight fan fiction from an unknown author on Smashwords who was certainly not a well known, filthy rich author at the time. The book wasn't my cup of tea, but it seems to have worked out fairly well for the author.