Because if you're sitting sedentary writing code and your heart rate is 90+ you're going to have a rather short life of writing code. That's why you should care.
Getting closer to retirement here. I've done my share of coding, and the heart rate hasn't killed me.
On the other hand, I don't spend large parts of my life exercising and worrying about my health. Why spend 30% of your off-time to perhaps live 20% longer? Especially if most of that time is going to be in a retirement home, worrying about bowel movements and whether there will be pudding.
I honestly prefer to live my life in the fast lane.
Caffeine, nicotine and endorphines keep me living when I'm alive, and still young enough to enjoy it.
If I die earlier, so be it - being dead isn't anything to fear. It's just non-existence. I was in that state for billions of years before I got born.
And now it's time for another pot of coffee and a fresh tin of snuff. Got another book chapter to write.