Why are text messages limited to one device at a time but phone calls are not?

I don't understand why Apple does not offer an option to have notifications (haptic and sound) appear on both the phone and the watch at the same time. The only way to do it is to turn off "wrist detection" which pretty much disables about 50% of the useful functions on the watch and it is no longer secure. Incoming phone calls will notify you on both the phone and the watch, so I know it's possible. So why doesn't Apple provide the functionality to notify you on both devices simultaneously?