Mage items should not give defensive stats

What is the reason and give defensive stats? I really want to know what the reason for this "balance". The build path for hourglass is probally the most cost effective in the game and it gives you almost everything except mana, which you get from ludens anyway. Where is the trade off for building ? you get HP, you get armor, you get the best active in the game, you get mana, you get CDR, you get AP, and you get Grevious wounds.

??? I really want to know.