Drivers Blocking intersection

Can anyone explain why drivers around here habitually block busy intersections?  3 nights in a row this week, people heading south on Fairfax County Parkway were blocking the intersection where traffic coming off the Dulles Toll Road could not proceed.  It was so bad, we sat through 3 light cycles where no cars could proceed off the freeway!  After that on the next cycle, exiting cars had to funnel to 1 lane so some could squeeze through.   I'm sorry, but drivers who block the intersection in heavy traffic are truly selfish individuals who who are just so impatient that they can't wait at the intersection line for traffic ahead of them to proceed.  Where is the common courtesy from fellow drivers?