Role Itemization

So is the difference in itemization based on role poor balancing or designed specifically for that and just hard to balance?

To elaborate, overall Bruisers : Strong kits Assassins : Strong kits Tanks/Support : average kits Marksmen : weak kits Mages : Strong kits

AP : weak itemization AD : strong itemization Tank/Support : average itemization

(i group tank and support together, because while they aren't the same their goal is, control where the damage of both teams is directed)

Personally it bugs me how being an AD bruiser, assassin or mage gives you both top kit and item power. Obviously with 120 champions there are some exceptions to this, who either have insanely strong kits to make up for their lack of itemization or weak kits which itemization can't make up for.