I dps because the healers in this game have no other option to do anything else but dps when your party is topped off. We don't have any significant supporting skills or any debuffing/buffing responsibilities so the only other thing I resort to when I don't need to heal is dps. I mean I've been playing healers and mmorpgs for about 7 years now some get them right, some don't. This game in particular could use some work on the healer roles and give us some responsibility other than iheallol.
How about halfing the potency of all the Stone skills for whm as an example and give them a debuff that lasts for 5 or 7 seconds that decreases the enemies defense or magic resistance by 10%. Or an insta buff that increases raid offense by 10% that lasts for 20 seconds with a 15 second cd?

Short version: The reason you see so many healers doing nothing when everyone's topped off is because there's nothing do once everyone's topped off besides dpsing.