Lack of proper mana sustain

Is it part of the "vision" difficulty? Why is there not a single flat percentage mana leech node on the tree? Or jewel mod? Do devs think its fun to manage your manapool with mana flask on attack builds? (Have no idea how it is on casters, I assume very simillar). My LA deadeye basically cannot kill endgame bosses without Thief's torment, since there isnt any other proper mana sustain method, and my mana dissapears within seconds including all flask charges. Thiefs torment is 5+ div, and i have to remove both of my good rings to swap in thiefs torment. Extremely frustrating.