That's part of the problem but the biggest part is actually the predictability of damage.
Encounters are so tightly scripted and highly predictable that damage can be anticipated to the exact second. and thus it's a joke to heal.
One of the reasons people say sch is op isn't because of its fairy. I mean ok sure it helps. But its lustrate/indomnibility that are generally considered OP.
A scholar can hit Aetherflow and due to scripted and predictable fights know exactly how many stacks he's going to need for heals in the next 45 seconds and exactly when they'll need them. The result is they then know exactly how many stacks they can turn into dps instead of heals and never need to actually cast a healing spell..
Astrologian is largely the same with many of its heals. Where they can literally time early dominance to pop a second after a raid wide attack goes out. and a similar thing with ED and even aspected benefic to a slightly lesser degree. The tanks won't take any real damage for at least 45 seconds The regen effect is more than enough to fully heal them..
If you want healers to heal more and dps less. Simply make encounters less predictable and less scripted... Bam that tank got smacked and I have no idea when he'll get hit again.. Best actually cast something...
honestly the highly predictable and scripted nature of encounter is what causes many of the games problems not just healers. tanks would use tank stance more if they weren't sure when or how often they were going to get smacked around.. but when its as predictable as it is. its like oh ill get smacked in 5 seconds pop rampart. there we go job done for the next minute...
even dps to some degree. blms popping triple cast for a minor dps increase because fights are so predictable they know they wont have to move much in the next 60 seconds or something like that..