If I run, I can skip leg days.
The belief is that, when I run I’m working my legs so if I do upper body in the weight room, then I am 100% covered. In truth, your running is mostly stimulating your body’s muscle fibers which are designed for endurance. The fibers that would train your body for strength go largely unstimulated. Without weight training, you may wind up with chicken legs or worse, motion imbalances.