It's largely a matter of gold efficiency. For the sake of comparison, these are the value of the stats on the items without considering their actives:
- Zhonya's Hourglass: ~87%
- Hextech Gunblade: ~120%
While the active on Zhonya's Hourglass is quite useful against Zed's ultimate, you are still at a disadvantage stat-wise in a fight.
The second issue is a matter of stat synergy.
Hextech Revolver provides Akali with a significant boost in damage which may not be immediately apparent.
It provides Akali:
- Additional sustain
- 15% healing from damage dealt (including your on-hit damage)
- 6.67% SpellVamp from her passive
- Higher autoattack damage
- 13% more of your AD will be dealt as on-hit magic damage
- +40 AD (increasing your on-hit damage further)
- Sticking Potential
- It's active provides a slow
- Burst damage
- It's active deals damage that scales with your AP
This is massive for Akali. This item provides a significant amount of damage considering you basic attack after Shadow Dancing to a target and to proc your Q, lets you stick to and burst targets by slowing them, and provides about 22% SpellVamp and 15% Lifesteal for sustain.
If you have this item, you can easily jump onto a squishy target and burst them down before backing off to quickly heal from minions. If you had this against Zed, you would have significantly more sustain than he does and any trade in which neither of you died would be highly in your favour.
Even if Zhonya's Hourglass is useful against his ultimate, it doesn't particularly outweigh all of the above factors. If necessary, build Seeker's Armguard -> Hextech Gunblade -> Zhonya's Hourglass.
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