Why is it so difficult to get work completed, and correct?

Three reasons:  (1) market demand, (2) shortages, and finally (3) attitude.  When market demand is high the second reason becomes commonplace.  Shortages in material and labor can be devastating to a construction budget and completion date.  Since construction has always been cyclical in nature, suppliers are reluctant to expand production facilities in fear of a collapse in the market.  Labor contractors are reluctant to expense money toward recruiting, and training, if they may looking at the possibility of layoffs.  An example of poor recruiting is that the average age of masons is now 50 years old, and with no one willing to enter the trade.  With too few workers, and small outdated production facilities, market demand will devastate any project.  

Third and most problematic is worker attitude.  "Its not my house" syndrome is prevalent across the spectrum, especially during high market demand.  Employers are eager to make money, at the expense of quality, hiring untrained workers to fill space on a job.  Unable to properly supervise the employees, the work gets completed in a very haphazard manor.  Untrained and unskilled the workers have no pride in workmanship.  The pressure on the general contractor to complete a quality project can be insurmountable.  That is why it is best to hire an experienced hands on builder even if it will cost a little bit more.

a quality custom home and remodel contractor.