Because, as I said, it could be a small heal. Like the actual spell called "heal" we have right now.
With the numbers you used as example those would mean absorbs of 10k per disc priest, 70k in total, from 7 healers who would do nothing else between hits but cast those absorbs. Even if they can cast those shields in 2s and the boss hits every 3s it would not be enough.
Now suppose he does anything that might deal damage to anyone in the raid on top of that. Would you still have all your healers be disc priests doing nothing but keeping those shields on the tank? You would loose the tank in less than 10s.
Now this is not really what i exspect fiights to look like in futur, but you get the point. Its not like your scenario is probable. Do you think they would design an encounter where the damage per hit exceeds the maximum health of the tank, but not the maximum health together with one absorb 2 healers out of 5 bring. I don't. (Priests and palas have it, druids, shamans and monks most likely wont have it.)