no reputable agent deals with unknown screenwriters
You clearly do not know much about screenwriting!
All of the major agencies do not accept submissions from unproduced screenwriters.
That's why at the film festivals, a number of screenwriters have collaborated with newbie producers, produced things themselves, etc.
A lot of production companies with big names do not accept submissions from an agented writers. Studios never do.
People like Spike Lee, before they found distrubtion, showed cased their work at film festivals (he wrote, directed, produced ,and acted in all of his early stuff).
Also, a person could send a query letter about a horror screenplay to an agent or a prodco, only to be rejected because all they deal with is dramas.
Agents find the screenplays much more marketable if the screenwriter has been produced, won an award, or otherwise already made a name for himself in the entertainment industry. If Justin Timberlake writes a screenplay, it's a sell (he's already written a book).