But why do AP champions need items equivalent to AD items when most of them have more range than AD champions? Sure marksmen are the exception here (as always apparently) but fighters and assassins have to get into melee range most of the time (some exceptions may apply).
Most AP champions (or maybe just mages) have the range to interact from a decently safe distance. That why (IMO) AD get a lot of decent MR items because they need to survive closing the gap. If AP champions had the equivalent for every tool, then the game would be heavily stacked in their favor.
I mean, have you ever tried fighting someone with more reach than you? You either need to outrange them, hit them faster/harder from the same range, or close the gap to hit them. So it comes at no surprise that AD (mainly melee. We all know how marksmen can abuse items not meant for them) get gear and abilities that help them close the gap. Because they are _melee_ and need to survive to fight in close range.