Because VIPER dont need it literally it really just dont need it, compare Bloodbath with Viper to the other Melees and you will see what happens, since everything viper has is physical and works with Bloodbath, pop Bloodbath and use Reawaken you will see your HP goes from 0 to 100


and if you die while not doing anything wrong its anyway the healer fault so viper really dont need any other defensive cooldown