For example, one rest day, one long run, one session of squats and deadlifts?
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?