Because using an amulet which has iceberg doesn’t mean you can’t get another item with a berserker talent, as I said, and i’m gonna repeat myself : you’re not forced to play a build with all of the talents from the same tree.
For example my wizard is using : comic, amplify, astral and empower, which are : elementalist / elementalist / elementalist and arch mage. (also I could be stronger with “focus” which is also arch mage, and get anything else instead of “astral”) so 2 arch mages/2 elementalist
Also, some of my items have affixes which are uselss on them BUT useful on another item, that’s the point, you have to take in consideration that you don’t have a single way to play and that the combinaisons are better than focusing on a single way/tree.
There is the fact : you want a high Damage and a high survivability. You can’t get everything, you’ve to make some choices!
When I started the game I was like you (and I guess like everyone else) : seeking for the best damage output and dying at floor 200 with 1 or 2 hits
So I started to rethink and rebuild my stuff to be powerful but without being as weak as a twig 
[quote]How is not random if i have to tap into three different skill trees to get a dps warrior, or a tank warrior ? Having and item with +x all skills is useless.
I mean no offence, i have looked into the game mechanics and skills but they are very vague and general. I would really love to know all the mechanics of the game to the smallest thing[/quote].
Warrior is the only class I don’t wanna play (I mean, spending time on him to stuff him correctly), but he’s wearing a tank build from random pieces found all over the floors, and he’s not dying at floor 200, he has : hardened (champion), phalanx (defender), bulwark (defender), iceberg (defender)
You’ve to take in consideration that “champion” would be more soemthing like a “paladin” which his tanky but can deal dmg and heal himself, defender is a pure tank, and berserker is playing only about the bleeding dmg (kind of DoT, which is the equivalent of the wizard elementalist)
Also you’re playing DQ, not WoW or LoL, the talents trees aren’t “dps, support, tank” or “dps, support, utilitaries” …
You can find a “tank” talent in a tree that would be more “dps” than another one, but that’s just because trees are more or less balanced, you won’t find every “dps” talents in a single tree, every “tank” talents in another one, etc… which would be stupid as fck 
Now, you can’t know in few weeks the whole mechanics and the “best” build, that’s a fact. Even myself after months of playing I still discover combos and better builds, but for that i’m spending more time on the theorycrafting than ingame.
And from what I read on the forum, a warrior can hit easily at 100m per critical hit without dying in a single hit at floor 200, so I think you can make a viable build which would allow you to survive and to deal ton of damage 
Wtf, you didn’t read my explaination or you just don’t care?
that’s one of the 3 best affixes in this game, +all talent / +all skills and +setnumbers … Read their effect please! 