You cannot kill someone while they have an absorb effect — it's like an invisible second health bar to chew through.
Healing is always doomed to be reactionary. The damage has to occur first, then be healed back up. No matter how amazing your clarivoyance is, you still have to allow the hit to come in before your healing power can do anything about it.
This makes absorbs inherently safer territory because it greatly expands the number of events that can occur before the target is guaranteed to die. It also slows down the required reaction time, by "stopping time" in a fashion — the player's health bar ceases to change during an absorb effect, whereas it can and will continue to bleed off while you wind up a healing effect.
If you can cast a heal which does 1 million, you can restore anyone to full health. Say your tank has 750,000 total health. In your hands, they're immortal — as long as you finish casting.
Now say they take 500,000 damage and you've got your badass shades on and you saw it coming and you're already 80.25% through casting your 1 million heal cast. Then RNG does your mom and they take another 300,000 unexpected damage. Your 1 million megaheal just landed on a corpse.
Now let's say you can cast a pathetic little 400,000 absorb bubble and 300,000 direct heal. That's like barely half your tank's health bar! You are a pathetic healer, and no match for the 1 million megaheal caster!
But in the same situation what happens? The tank takes 500,000 damage, were already bubbled, and so their actual health only drops to 650,000. Then that 300,000 unexpected damage comes in — dropping them to 350,000. And your sad 300,000 heal with sluggish reflexes comes in and they're back to 650,000.
That's why absorbs are powerful. The example is contrived and overly simplistic, but the central idea is the driving point: heals are always stuck undoing damage that has already occurred, while absorbs simply stop it from ever happening in the first place. In an ideal situation both are on equal terms, but as healers we're really not concerned with the ideal situations, right? It's always the non-ideal ones we're stuck cleaning up. ^.~
The devs know this and this is why absorbs are much harder to produce than heals, have longer cooldowns and more complicated mechanics. In a head-to-head battle, absorbs would always win if all factors were equal. But absorbs have a powerful niche place instead, since heals are plentiful and readily available while absorbs are not.