I think it's a matter of energy minimization. It simply doesn't make sense for the heart to beat so fast when your body doesn't need the huge amount of oxygen and nutrients the heart cycles through the body with the advanced speed it operates.
The heart -- a muscle -- may actually suffer damage from sustaining contraction at max speed at all times.
As research on excessive endurance exercise and its effect on the heart shows,
Heavy and sustained ET [exercise training] generates large quantities of free radicals
that likely outstrip the buffering capacity of the system, leaving
these individuals susceptible to oxidative stress and transient
Potential Adverse Cardiovascular Effect From Excessive Endurance Exercise
Not to mention, the heart operating at max speed will not pump enough blood to the organs and extremities, actually reducing blood flow to throughout your body as compared to a heart with variable heartrate.
Supraventricular tachycardia, a heart condition that features episodes of an abnormally fast heart rate, is a short-term example of your 'max heart rate' hypothesis.
According to NHSinform.scot,
In SVT [Supraventricular tachycardia], the heart muscle is contracting
so fast that it cannot relax between contractions. This reduces the
amount of blood being pumped around the body, which can make you feel
dizzy and short of breath.
And sustained tachycardia (>= 30 seconds) is especially dangerous for the human heart.
It [sustained tachycardia] may lead to more serious ventricular arrhythmias, such as
Ventricular fibrillation involves sudden, rapid, irregular, and
chaotic heartbeats in the ventricle. These erratic electrical
impulses, sometimes triggered by a heart attack, cause your heart’s
ventricles to quiver.