Why do CMOs last only 20 months?

Tim Pollak argued that the Chief Marketing Officer needs to have clearly defined responsibility, otherwise he's made a scapegoat for things beyond his control. He said that unless the CMO is also responsible for product development and sales, he isn’t actually accountable, unlike his other “C-suite” counterparts.

But is this just an excuse for the fact that most CMOs don’t seem to be up to the job; indeed, is marketing even a “C-suite” role?