Hourglass is so bad, abyssal + seeker's armgaurd is much better even when laning against ad.

Abyssal + seeker's armguard gives 105 ap, 50 MR, 45 armor, 20 mr reduction for 3550

Zhonyas gives 100 ap and 45 armor for 3500

The 20 mr shred helps you do a lot of bonus damage to minions, and more dmg to champions too. Zhonya's hourglass is only useful in like team fights or when getting turret dove, it doesn't help you to kill the opponent or in 1 v 1's in general (ad champions have no cooldowns)

I believe building a full legendary item should always be more cost efficient but this clearly isn't the case.