Experienced bodybuilders often use the four day split, because in this way they can train fewer muscle groups per workout, giving each of them sufficient attention and allowing training with higher volumes. (fitnessandpower.com)
There are very few muscles that stand out like the traps. (fitnessdoctrine.com)
So, in running, your body has to distribute the same metabolic power across a lot more muscles, while in biking it can pour that same metabolic energy into relatively few muscles. (philmaffetone.com)