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A pedantic question: the answer being, of course, that 'writers do many things’: inspire, castigate, reveal, enthrall, inform, inflame, and entertain. More importantly, they get to the point. “Brevity is the soul of wit,” as Shakespeare says: a euphemistic way of saying “don't waste my time”.
If you are a writer, whether your choice of media is the novel, poetry, screenplays, drama, or even academic, you write for an audience. Your relationship to them, while important, is beside the point. You may in turn love, hate, fear, or desperately desire to please them, but the fact is that the audience has given you the most precious gift that anyone can bestow upon another: their time. Time that could be spent with children, cultivating their own works, making love, or simply time for themselves—the audience expects value for the cost of admission!
Too much buildup, finessing, static exposition, and by the time something actually happens, the audience has completely forgotten what they are even doing there, assuming they are still there. Note that this has almost nothing to do with how long the work actually is. Whether the piece is a twenty second commercial, a 1,000 page epic novel, or a comic book, the prerequisite is the same: the audience needs to know what the character relationships are, who wants what, and what's at stake if they don't get it. Everything else builds out from there, shaped by the rules of the world, the conflicts which naturally arise from their desires, and the obstacles within the world which bar them from the object of their desires. The scope of these conflicts determine how long your piece needs to be, though not necessarily how long it is.
Two examples:
1) The cartoon rabbit desperately wants to eat a bowl of fruit-flavoured cereal; the kids have said cereal, and intend to keep it all for themselves. Ergo, the rabbit must endeavour to steal the cereal, but is thwarted time and again by the kid’s machinations. Silly rabbit—The End.
Cut and dry. The kids don't enter into negotiations with the rabbit about whether or not they might be willing to part with their cereal in exchange for some service or other. The rabbit does not file a lawsuit against the kids, suing for damages, nor undertake the task of getting a job to be able to buy his own. White hats, black hats: done.
2) The Prince of Denmark has returned home where his father has been killed, his mother has married the uncle, and the uncle has seated himself on the throne which should belong to the Prince, who is a mature man. Something is very wrong, and the Prince is going to figure it out, no matter the cost, to himself or to others; his primary adversaries being those with whom he should closest to. Maybe he's crazy, maybe he isn’t, regardless, it's about to get very messy, and will take some time to unravel completely—about four to five hours, in fact. Ironically, one of the primary critiques of Hamlet is his inability to get to it already, but this is in part the essence of the drama’s delayed catharsis. We might excuse the Prince for not simply murdering his entire family outright, no questions asked, at which point his mental stability would not be at all in doubt, and besides, there would be no play.
Human beings, as far as we know, are the only species that tell stories. Our existence is wrapped up in the story of us. To be a storyteller is a noble and demanding trade, asking much of both teller and audience—this is an exchange to be respected on all sides.