Because poets are filthy, filty people, that's why.

Coming from the cesspool of Smalltown, America, I've often smelled the reek of willful illiteracy, a stench so strong it could choke a duck. Ducks are smelly animals, by the way. While I would love to combat this chosen ignorance as a whole, it would probably be more worthwhile to conduct sorties against specific stupidities. I will begin by addressing the inanity espoused in the question, which I once heard and I'm almost certain you have too, "why don't poets just say what they mean?"


This is where I grew up.

If we responded, "because they're artists," or, "because they love the craft," or anything similarly esoteric, we'd be speaking a whole different language. We cannot communicate with the dunce on our terms any better than we can understand why they embrace the Blue Collar Comedy Tour. What we do, is we address their base understandings and make the reasoning practical. To point, we wouldn't tell them music uses a minor key to elicit feelings of dread. You say, "sharp violin sounds put you on edge." The person will agree, because violin makes them nervous no matter what, since you never know what those artsy types will do next.
So, let's take a look at poetry and figure out why those queers (odd fellows) like to use such flowery language when they could just say, "you're pretty. Nice weather is also pretty."
I remember a classmate of mine, when reading aloud (which we still did in high school, did you?) mispronounced "tidy." It was not intentional, maybe, but either way it came out "titty." We all had a good laugh and got on with our story, which was probably Mort D'Arthur and probably had something to do with shriveled up "duggs."


My search for "duggs" was surprisingly fruitful.

Now, imagine if you were a poet, and you wanted to get away with telling a dirty joke, and you live in Renaissance Europe, when poetry was probably at its peak. You live in an era in which women can't be on stage, you can't belong to a church other than the monarch's and everyone listens to astrologists because they apparently clung to stuff about not having sex before marriage but didn't get around to Deuteronomy 18:9-11. So, it's not so different from today.
In spite of all this, you want a dick joke in your work. Shakespeare was the man for this. To grab an example from Michael Swaim:

Hamlet: Lady, shall I lie in your lap?

Ophelia: No, my lord.

Hamlet: I mean my head upon your lap?

Ophelia: Ay, my lord.

Hamlet: Do you think I meant country matters?

Ophelia: I think nothing my lord.

Hamlet: That’s a fair thought to lie between maid’s legs.

Ophelia: What is, my lord?

Hamlet: No thing.

Ophelia: You are merry, my lord.


Which doesn't sound so bad, and that's what ol' Willie Shakes was counting on. Swaim goes on to explain that "country matters" is a wordplay for "cunny matters." Hamlet is talking about putting the tip in, how awesome it is and Ophelia responds by telling him what a horndog he is.
This sort of "slipping it in" wasn't just something for the 1600's bullshit piety. You can look in the 20th century and find it, like in e.e. cummings "she being Brand." I mean, he could have literally "oiled the universal joint" or meant "slipped the clutch," but I doubt it.
On a related note, you can use poetry to talk about some gross stuff that is, theoretically, acceptable, but in a more palatable way. Donne is a good example of this in his poem "The Flea."

Mark but this flea, and mark in this,
How little that which thou deny'st me is;
Me it sucked first, and now sucks thee,
And in this flea, our two bloods mingled be...

At the time, poets used conceits, which is a term for metaphors strethced so thin you could say they were, "condoms over broomsticks." That's a conceit, more or less.


"stretched condom" did not yield great results.

One of these conventional ideas was the idea that sex involved the mingling of blood and this idea was accepted as scientifically sound as the idea of chemo therapy is today ("What is this?" asked Bones. "The Spanish Inquisition?"). What Donne is talking about is, a woman turned him down, but they practically boned because this here flea sucked them both. It was socially kosher, but still weird.
Another good example is his "Song," which reads,

Go, and catch a falling star
Get with child a mandrake root
Tell me where the past years are
Or who cleft the devil's foot
Teach me to hear the mermaids singing
Or to keep off Envy's stinging
And find, what wind
Serves to advance an honest mind

To put that in common English, "here's a list of impossible shit to do. While you're at it, could you tell me how to make bitches not cheat?" Everyone back then would agree with this idea, but just because you believe it doesn't mean you say it. That's rule #1 of prejudiced humor.
Saying what everyone's thinking is a long tradition of poetry, and sometimes everyone's thinking "wow, that sucks." You can talk about some mean stuff with poetry, and Carl Sandburg saw a lot of mean things in the world. By the way, he grew up about twenty minutes from where I did; and he hated it.


Even though they named a mall after him. Whadda douche.

Sandburg's Chicago Poems is one of the best volumes in American literature, representing the Windy City as a brazen, sublime, tormented place.

GOOD-BY now to the streets and the clash of wheels and
locking hubs,
The sun coming on the brass buckles and harness knobs.
The muscles of the horses sliding under their heavy
haunches,
Good-by now to the traffic policeman and his whistle,
The smash of the iron hoof on the stones,
All the crazy wonderful slamming roar of the street--
O God, there's noises I'm going to be hungry for.


That's from "A Teamster's Farewell" and it's a pretty moving piece. Now, your average journalist might have just said, "another mob peon was arrested and it sucks to be him, locked in a cage for a couple of decades." That journalist might also wake up dead, which is to say, not at all. Sandburg frames it with verse and makes it something to discuss. As an added bonus, he died with his lungs inside him.
Which is a lot of why artists do what they do, how they do: no one disembowels artsy folks. This is mostly because people aren't sure if that will kill us weirdos.

-Black Ranger (rhymes with "sack danger")