Part of the appeal of healing is that you can save people. It's in the name of the role. Removing that with overly frequent one shots puts the burden on DPS but removes that aspect of the healer's identity.
Some one shots are good variety in fights. Too many means there's little a healer can do to sway an outcome by doing something exceptional and saving someone who should have died. An example is Hashmal's extreme edge. That rarely one shots people these days damage wise, they usually die from the DoT tick. That means that while it's the target's fault if they die, a healer who sees it fast enough and is in range can save them from that death. That's better than just "oh here's 2 million damage you die instantly" because it's clear both who made the mistake and also gives the healer an opportunity to intervene.
To me, that's far more compelling healer gameplay than "nothing to do, spam another Stone".
Except that Cure does things like Freecure and Lilies. Freecure used to matter but doesn't because MP is a joke now, and Lilies are irrelevant because of what they do. If those things were relevant resources, Cure healing has more going on mechanically than Stone spam does. Plus, I'm probably also using Regen, Tetra, Asylum, etc.(3) Well, the thing is that if you want say 70-30% heals, you are essentially asking people to cast cure as much as stone in a fight, so you would just swap the boredom. Rotations tend to make that somewhat more interesting and probably would end up being made.