Because in proper manuscript format for a short story, thirteen lines of the story is the maximum you can cram onto the first page.
Several years ago, I was on the GEnie online network, and participated in the Science Fiction Round Table (SFRT). Science fiction author and editor Damon Knight also participated, and in one forum he agreed to allow people to post the first thirteen lines of a story so he could comment on them.
The first thirteen lines are the most important when submitting a short story to an editor, because if he’s not interested after thirteen lines, why will he even bother to turn the page before rejecting the manuscript?
So, that’s why I chose thirteen lines per day as my goal.
That doesn’t mean my thirteen lines will always be the beginning of a story, of course.
(And yes, I tried posting thirteen lines a few times for Damon Knight to read. Only once did he react positively. And I don’t know where those thirteen lines are now.)