When these training gospels were devised, most runners had to work, or at least study, so their routines had to be built around a 7 day timetable.
There wasn't much scientific at all about these 7 day plans. I doubt very much there is some biological reason why a human needs one day a week rest from training, rather than say 1 in 10 days.
Apparently we adopted the 7 day week, because the number seven had mystical connotations for the ancient Babylonians, built around the fact that there was (known to them) 7 heavenly bodies in the sky.
So not much scientific basis for assuming that a runner benefits from one day a week rest, or one long run a week.
Most elite runners today can run and train when the hell they like. Speaking personally as a hobby jogger, I can also train and structure my routine as I please, because I work entirely from home. So tell me coaching experts - why should I be sticking to 1 long run a week, rather than creating a 10 day timetable, or a 5 day timetable?