Maybe it's because they thought 'hey healers should be about healing, let's make them have a mechanic where they have to heal to solve it' and then realized that actually, there's quite a few players that aren't great at the 'healing' side of healing, be that running out of MP, not using OGCD heals correctly, or the greeders ignoring things to chase parses
I jest, but only partially. I also wonder why they haven't done such a mechanic more often, my guess is something akin to 'it's hard to balance such a mechanic when there's such a vast disparity in how much healing certain healers have access to, and what profile that healing takes'. For example, if it were a mechanic where the healers have to fill one target's HP bar, Benediction is an 'I win' button and instantly solves the mechanic. If there's multiple targets to heal (like Shinryu EX), and they all start at 1HP, then Essential Dignity skyrockets in value due to its 'missing HP' scaling thing. Half of the healing tools would not affect such targets, due to being AOE (unless some very specific coding took place), so some healers would suffer more than others (eg SGE being able to Pneuma 4 targets vs not being able to is quite a hefty difference).
It's why I've advocated for 'shield checks' to be a thing (by giving all healers access to AOE shielding with varying degrees of effectiveness/accessibility, as an inverse sliding scale to their 'pure healing' capability, so being good at one means being not-good at the other), and those 'shield checks' to be binary 'is a shield present, yes/no' checks, rather than 'you need XYZ HP worth of shielding to successfully resolve'