Because it would defeat the purpose of the poem. Poetry seeks to embrace the medium of language and play with its possibilities, ultimately to get an emotional response out of the reader. You can't do that with a few blunt, simple sentences.
It's like saying, why play music when you can summarize most every song with the words 'Verse, chorus, verse, chorus, bridge, chorus'? Because music makes you FEEL something, and that feeling is really all that matters.