system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If cardio is done by raising the heart rate to a high level and sustaining it, why does the heart not constantly improve its health since it never stops beating?
gpt
If your heart needs to get stronger it will, that usually means you are in trouble. 

Look into the physiological mechanics of most cardiac problems and you will see that most of them aren't the heart muscles failing the body, it's the other way around.

High Blood pressure? Your heart muscle is fine, the problem is that it is working too hard because your body is in bad shape, narrow arteries, obesity, reduced kidney function. It will keep working too, but your arterial walls aren't strong enough to withstand the extra pressure and over time will weaken. 

Heart attack? That's a clot or blockage in an artery that starves your heart of oxygen, again it is your body failing your heart muscles.

Low blood pressure? Hormone imbalances mostly. Your heart muscle is normally fine but it keeps getting orders to beat slower and weaker than it is capable of. 

Your heart muscle is so amazing compared to the rest of your body that managing your cardiac health is mostly about getting your body closer to the efficiency of your heart to make its job easier.

Cardio expands your blood vessels and improves your circulation. It improves your bodies efficiency at using oxygen and dealing with the by products of anaerobic exercise. It means that your heart gets to take it easy.

An incredibly healthy cardiovascular system is one you are likely to find in people with low excess body weight, active lifestyle, and a good diet that doesn't tax their metabolic systems. This hypothetical person will have a slow heart rate with a strong "beat" but not high blood pressure. Because the body is up to snuff and will have vessels large enough to allow blood to flow easily and a body efficient in using it.    

TLDR; You heart is so powerful, efficient, and sexy that it mostly doesn't need to improve, it's your scrub-ass "everything else in your body" that fails your heart.