Could also just be a case of healers tunnel visioning while DPSing and/or waiting for oGCDs to get off of cooldown.
I'm a Green DPS, I like spending my cast times DPSing and leaving heals to oGCDs. There are a lot of people with this mentality and the game does kind of foster it.
We are also so far into the expansion that people are going to try different roles. With Tanks and DPS both having more DPS-focused gameplay, never really having to look at/worry about other partymembers' HP (sans a handful of mechanics), that mentality might bleed in while playing healers.