While you're correct the two are very closely related.
Dunno about you, but many players I encountered who dropped healing, refuse to take it on, or are very slow to do DF entirely on their own acted this way because they find the healer role stressful. Either from their own experience and/or watching what other healers deal with.
And I don't really think much can be done about it unless SE do something like significantly increase the amount of gated trash to stamp down on chain pulls, make holding aggro next to impossible without a tank stance or make it mechanically too punishing for healers to ever be expected to dps. I don't think I need to explain how huge the backlash would be if any of this happened.
That isn't to say I haven't come across players who simply didn't find healing mechanically engaging or fun, just saying I have come across many more who simply don't want to deal with the stress of the role.