Why do so many people build Morellonomicon when it's bad?

Since building morellonomicon actually lowers your damage since oblivion orb gives the pen and most of the hp and unless you need the anti heal, all morellonomicon gives is ap. Lets lets do some simple math here: After buying oblivion morellonomicon cost 1400 gold and you get 50 ap Now instead of upgrading to morellonomicon you can just get a fiendish codex and a amplify for basically the same price but you get 5 more ap and you are building into a item that gives you something useful zhonya's.

Now if you need anti heal then yeah build it but in most games you don't need anti heal but people still buy morellonomicon I DON'T GET IT since it's bad like really bad. Like late game it can just get replaced for void since they would actually reduce pen by similar numbers.

Can someone please explain why people buy do this please?