Insta Kills

I know this may seem like a dumb question by some so I am only Interested In helpful replies. I want to know what's up with all the maddening Insta kills? I understand that we must be geared and should be aware of what we are up against In any given area so please refrain from telling me that. Thing go south quickly In a lot of spots where sometimes I don't even know what killed me. Seems to me that the dev's want use to die over and over and are quick to limit how much damage we can do. Why is it so lopsided In the mobs favor? Why can't we gain and stay good why are we always killed and almost never seem to gain any type of advantage? I am really Interested In any who would reply their good thoughts and comments on this subject. Please lay it on me so I can better understand the game and make decent progress, thanks to all In advance for your help.

simple you either run a high ES shield build in the mid levels or you somehow got a very good weapon to one shot everything an randomly die to white mobs here and then.

i got the second type where my ranger one shots most mobs in t15s but i randomly flop over to something i cannot dodge or see... it is fun you run up a stairs and 3 rolling mobs roll over you at warpspeed.
80% evasion with that all you dodge node thingy.
thing is i only had currency to buy a semi decent crossbow and a decent armor with 2.2k evasion, but rest of my is not good. nothing drops and i have 3 chars above level 87 running t15s in glass canon mode and i did not find a single divine orb.
i also didnt find any good gear so i can only continue to play glass canon or i just quit.

my infernalist has 7k ES and rarely dies but has terrible ST dmg, so thats the trade off...