It's not that we don't know what to write, it's that we don't know where to start.  
It's  as if the bewilderment of choice overwhelms the decision-making  circuits somehow.  The raucous din of 1001 ideas and desires and fears,  all clamoring for expression now Now NOW!  A man with 2 watches never  knows what time it is and frequently misses the train.
But  all that racket and hooplah just won't do.  To write effectively, we  have to drill down into the very word and phrase of it, take the idea's  wings and polish them to shine, then fly it seductively into the gaze of  their mind's eye to grab their attention.  
The dance of meaning must be precise and properly dramatic, or one of  the other thousand nearby explosions of sensory input will drown our  words in the numb disregard of distraction.
The  glint and glare of unwritten ideas can wreak havoc with our ability to  begin, continue and finish with any semblence of sanity afterwards.   
We're  like sugar-high children at a carnival of possibilities.  All the noise  and magic and sawdust frenzy of the whole thing makes it hard to slow  down enough to do anything except marvel, and want to do everything at  once because it's all so amazingly alluring.  
If only doing everything didn't mean nothing would really get done.
The  other side of the same can't-decide is the slothful difficulty of  actually FINISHING something and giving it final public birth.  It's so  easy and alluring to let it loll around unformed and unedited, always  just about to be The Perfect Idea, The Perfect Piece.  Just  about....almost.  Almost never happens.
Unfortunately, creation demands we slay The Possible to birth The Actual. 
We  have to make explicit, defining choices over and over again till we've  soaked it in our intent and our vision.  And no one can tell us how to  do that but ourselves, because the answer is scribed inside places we  can't even see without the creation acting as a bridge into them.
But  so much of life is the direct opposite of that, a near-constant stream  of discouragement and jockeying for position with little regard for what  your say is.  A tide of No's and demands that you must adjust to.
Writing  is hard because it requires courage enough to simply have an opinion or  story.  But to take the time to nail that lofty butterfly down and tag  it with words so others can enjoy it is even more difficult.  
The  butterfly would rather flit free afield, far from the semantic eyes of  the reader.  And it's easy to get entranced simply watching it fly around. 
However, the net won this battle.  Next time it may go to the butterfly.  
Remember: you can't catch if you don't chase.
Start!
