New Player Here. Are Scavs ever this accurate?

Been following the game for a year or so before getting it over the New Year Sales.

In all the streams I've watch, the scavs takes a while to notice you then slowly turn to shoot at you. And once they start shooting I've never seen any streamer instantly get 1 tapped to the head and killed.

I'm saying this cause I'm getting my head instantly removed whenever I try to fight the Scavs. It feels really frustrating cause half the time I can't even see them. When I fight another player and die, I at least feel like they gave me a fair fight (they have to see me to shoot me, we both miss some shots, etc)

The Scavs on the other hand just speak some Russian Magic Spell and half a second later, my head just pop open. Is there any tricks to not get instant killed by Scavs?