No one here would know why Apple doesn't have such a feature available on the Watch. And, speculation about unannounced features is prohibited.
You can let Apple know that it's something you're interested in here:
Product Feedback - Apple
Personally, I wouldn't want Apple to release something like that unless they were very certain it would work. Imagine how awful it would be if the Watch didn't detect the heart attack and someone was relying on it and didn't go to the ER until it was possibly too late.
Until Apple should provide such a feature, make sure you and the people around you know the symptoms of a heart attack:
https://www.cdc.gov/heartdisease/heart_attack.htm