Basically the title. If I go to the gym three times a week, I can hit different muscles each time (meaning each muscle is really only being targeted once) and I'll see noticeable muscle growth over time.
If I run once a week, though, my heart won't improve in any meaningful way. I couldn't run once a week for 20 years and be ready to run a marathon, but I could hit each muscle once a week for 20 years and be a legitimate bodybuilding competitor.
What's the difference? Why is there no way to do one cardio exercise a week and see significant improvement?