There was an old Challenge magazine article years ago that suggested a pseudophysics explanation for the "every jump takes one week" rule. I can't recall the full details, but in effect each jump rating was a different energy state, somewhat like electron shells. The actual process of raising the ship to the required level, moving it, then bringing it down was explained as pretty much invariant.
However the real reason IMHO was to make it so that in-system use of the M-Drive would occur. If you could jump in-system for a proportional fraction of the stellar jump, you get almost instant travel between planets (interstellar distances are a LOOONG way compared to in-system ones. 1au is about 1/200,000th of a parsec.). Setting all jumps to take a week means that you will normally M-Drive to any destination you can get to in less time (and much less fuel)... which means more chance for encounters and other roleplaying moments, as well as another reason to desire a ship with thrust better than 1G.