For anyone who uses a heart rate monitor for exercise, it isn't a surprise at all that this capability isn't included. Detecting your heart rate on your iPhone button means holding your finger on the button, which likely means stopping your activity. Doing this will lead to lower-than-workout level heart rate readings and defeats the purpose. The best (and only way really) to get accurate, reliable, continuous measurement is with a chest strap that transmits to a watch (or perhaps to gym equipment).
Now, there could be some other uses for it, but it would be VERY niche.