1. sometimes they aren't looking or just do it preemptively, there are a lot of impatient tanks that charge in the moment they are allowed, out of combat mana regenerates really fast so its hardly using up any resources to cast it early.

2. Nervous healers? It could also be that they want everyone topped up constantly with Medica II so they can DPS more. Divine Seal+MedicaII/Regen will give you a lot of breathing room to comfortably DPS while keeping everyone healthy.

3. Either Swift is down (or they lack it) and they aren't comfortable not healing anyone for 7 seconds, or the other healer has died several times already and has proven to be a liability.

The first problem doesn't bother me at all, second one I find sometimes in trials and Alex raids, which is fine as long as they can keep their mana up and not cause hate issues. and third is really rare, if I die or someone else dies, a rez is almost always immediate.