I do not understand why GLP isn't purchased more often on mid lane Zilean

has THREE really good synergies with , yet almost no one buys the item on him despite him traditionally getting when mid.

It can:

• Set up AOE double bomb stun
• Give Zilean, whose full rotation does a meager 150-600+180% AP, an extra 100-200+35% AP damage, significantly increasing his damage output
• Allow Zilean to proc TLD without requiring an auto

Seems like it would be at the very least preferable to builds