Well that's because building defenses requires more situational thinking than building damage. When building defenses you have to look at what are the main damage sources on the enemy team, as well as what champ you're playing (ex. you might build Warmogs on Mundo but not other tanks). Meanwhile, when building damage it's mostly just what champ you're playing.
That isn't to say that there's a massive difference; no matter what you should always build according to situation.
As an AP, sure, you'll probably get the same items, just in a different order but it's still pretty similar for tanks, just with 2 different "paths."
Likely AP items
Morello's, Zhonya's, Rabadon's, Void
Likely Tank Items
VS AP heavy/fed AP: Banshee's, Visage, Merc's
VS AD heavy/fed AD: Randuin's, Thornmail, Tabis
LoL in general is pretty bad with this kind of thing. There's one way to do things and every other way is just not as effective,or only works well in specific situations.