Because if it's a moderate percentage wouldn't it be better to have a Wizard using this spell and rune have LESS maximum life? For example, if you have that spell and rune active and only have 1000 maximum life the enemies, hypothetically, could only deal 1/4 of your life per hit (250 HP). So if you pair that with a health regeneration per second of 400 or 600 that would essentially allow you to take a lot of hits, so long as you don't suffer too many in the same exact second before your health regen can heal you up. Plus if you combo that with a rapidly hitting attack and a powerful 'life on hit' effect wouldn't that allow you to potentially survive easier in and out of combat?
I apologize if there's something obvious that I'm missing here, but the thought just struck me as kind of strange.