The same reason people don't AOE or use other basic parts of other jobs- ffxiv players love doing the absolute bare minimum for battle content.

In my experience, when I ask these players why they aren't using X ability (no I'm not being mean about it, this forum loves to jump to that conclusion), the answer is usually something along the lines of "we're getting through it, aren't we?" or "you don't pay my sub."

It's just toxic casualism, most of the time. Not all the time, because there are people who just don't read tooltips, just most of the time.