Because the Forerunner is a pro running device and the Vivoactive is a fitnesstracker / allrounder device.
If you want to have pro running features, use a forerunner. You are a riding a bike, go for an Edge.
Everything at once like Hiking, Running, Riding, Swimming or Triathlon take a Fenix.
Fittnessfunctionality and a bit of everything (but not pro), use the VA.
For that, Garmin has the feature comparsion.
You also cannot expect sailing or pilot features on a (comperatively) cheaper device.
Guys I also have a vivoactive 3 music and I cannot get it to vibrate at all .. I have the vibrate turned on to hard and 'do not disturb' turned off (incase this may be having an effect) but no vibrate.. I need this to work to let me know when times are finished etc for workouts .. bizarre .. just a fault maybe ?