Probably the reason that seven is usually seen that way was originally for astronomical/astrological reasons. The 7 known moving bodies in the sky at the time were: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. In fact, our 7 modern days of the week are named after these different celestial objects.
http://en.wikipedia.org/wiki/Days_of...es_of_the_days
It's interesting to note that the Hebrews just numbered the days of the week, though, except for the Sabbath aka day 7. The calendar system with 7 days in a week was probably culturally transmitted to the Hebrews somehow from some of the other nations like the Sumerians or Babylonians -- some of which were avid stargazers.