You don't see 4 Dorans Blades because there is Lifesteal/AD/Health items out the ying yang. Lifesteal is a more prevalent stat that effects more in the long run, so sitting there wasting gold on the same item instead of investing in lets say, blade of the ruined king, is really bad. People build multiple dorans rings, not because of the item itself, but because mages quite literally suffer from lack of mana regen if they do not build Archangels Staff. ADCs or other people who build dorans do not suffer from lack of mana regen as Auto-Attacks/Energy are enough to sustain themselves off 1 Lifesteal item. And you may be thinking, there's plenty of mana regen items for mages. Well, lets take a look, Morellonomicon has a on kill/assist mana regen item, not in level(mid level regen) Athenes Unholy Grail offers mana/health regen however it's other passive is 100% useless since Spellvamp was technically removed from the game. The only noticeable mana regen items I can see are Rod of Ages, ArchAngels(Rarely built on most mages since it takes longer to build into Seraphs than it does to finish a game lel)
AD wise for Lifesteal there is:
(this one can go either way but spellvamp is arguably more useless than Lifesteal so it's primarily used on Assassins like Akali since it has both stats)
(This is in here cause it doubles all on hit effects including Lifesteal)