Is there a physiological reason? When asked this, the first thing I would think of is that there is less stress placed on the heart, however, I think that logic is flawed. If we wanted a lower heart rate because its less stress, we would also avoid exercise. Curious on your thoughts.
A lower heart rate is associated with increased fitness, which is the likely mechanism involved with reducing risk of cardiovascular event and all-cause mortality.
The way I understand it might be/probably is wrong, but here goes my way too simple answer.
Your heart is like a machine and like other machines your heart have an end-date. The faster it beats the faster it will reach the end-date.
When we exercise the heart rate goes up and this is probably where you think its flawed.
But when we exercise and get in better/good shape, the heart will be beat slower when not exercising (daily life, sleep and rest) which is the majority of your time. Hence - in the long run you heart needs to produce fewer beats all in all when you are in good shape.
No, this is not true.
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Would you care to elaborate and maybe educate us on how it actually works then Austin? 
I tried searching for some info and found this:
“A lower resting heart usually indicates more efficient heart function and greater cardiovascular health. As you train more, and your heart gets more efficient, it’s able to pump better, and you don’t need as many heart beats to put out as much blood each minute,”
Is that a valid explanation?
See the reply to your next post.
By increasing the cardiac output at any given heart rate, the heart can meet the body’s needs more efficiently, yes. The adaptive response is to reduce the resting heart rate in order to meet the body’s demands at rest.
As far as how this reduces mortality and risk of cardiovascular disease, it’s not related to a predetermined number of heart beats that the we have before it “wears out.” Humans are not like machines, but rather we can adapt, remodel, repair, and ultimately find a way to function well in a variety of different conditions. Thanks, biology.
It appears that the improvement in fitness and its associated parameters, e.g. VO2max, LBM, decreased fat mass, etc. all drive the improved health outcomes through a number of mechanisms such as increased physiological reserve, improved immune system, etc.
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Thank you very much for the detailed clarification Jordan.
Interestingly, though maybe a topic for a different post, there’s evidence that HFrEF patients’ hearts actually beat themselves to death to maintain CO and BP.
They will try to, as the brain will stop at nothing to sustain itself 
Well, after all, the brain is the most important organ in the body, at least according to the brain.
It’s kind of a dictator in that regard.
If we all had a finite number of heartbeats, then regular exercise should shorten lives, by “using up” more beats. In fact, we find the opposite to be true.