Why language understanding is not a solved problem

Over the years, periods of intense excitement about the prospects of machine intelligence and language understanding have alternated with periods of skepticism, to say the least. It is possible to look back over the ~70 year history of this effort and see great progress, and I for one am pleased to see how far we have come. Yet from where I sit we still have a long way to go, and language understanding may be one of those parts of intelligence that will be the hardest to solve. In spite of recent breakthroughs, humans create and comprehend more structured discourse than our current machines. At the same time, psycholinguistic research suggests that humans suffer from some of the same limitations as these machines. How can humans create and comprehend structured arguments given these limitations? Will it be possible for machines to emulate these aspects of human achievement as well?