Because it’s not Glare
/s

I always assumed it was because they wanted to avoid overheating from the regen ticking while target is at full HP. I don’t have White Mage levelled to 80), but I know as an Astrologian using Diurnal Sect that I can lean on them heavily if I need to, especially if I throw in Collective Unconscious / Celestial Opposition / Neutral Sect (and I’d assume it’s the same for White Mages)