high DEX gives higher defense rating, they are hard to be hit; rouges don't mean to be tank anyway, good movement and invisiblility make them easily move in and out of the sticky situation.
If they're going to attack anyone they need to get in melee range. Once they're in melee range they rely on dodge, armor, block, and raw hp.
Warriors have better access to armor, block, and hp. The only thing rogues have is a bit more dodge.
Backstabbing doesn't do as much damage as a 2-handed weapon. And rogues are gimped because they're limited to only piercing weapons - there's a lot of enemies that are resistant to piercing (most notably, skeletons).