Well, most of it is because healing power scaling. Meaning that healers are literally just defined by how much damage they give the party, because no-matter what you're going to have the healing in spades to not only cover what damage is done, but also to afford Tanks to sit in DPS stance all day
This is part of it, but part of it is also that tanks and healers are uber homogenized and don't have any niches or weaknesses anymore. Look at ARR SCH's ability to handle sustained AOE damage vs now and tell me that HW didn't completely throw the baby out with the bathwater in terms of healing design. Look at ARR warrior (even though post-2.1 it was still OP LMAO) vs PLD and compare to now.

Aside from aesthetics and a handful of playstyle differences, tanks and healers in this game handle all situations, no matter how different, with an equal amount of ease, and I think that really limits design, and frankly makes meta choices boil down to little more than "what makes our group do the most DPS".