What are the most common reasons people don't dps?

[miraidensetsu]

I do approach tanks and DPS who are obviously doing something wonky and try to direct them to the right direction, it's not just healers.

Generalizing all healers who don't DPS into a single category and attack them is not "direct them to the right direction", but bash them. Guide healers in the right direction is to give them tips when needed, try to understand why he is doing that and suggest some action. It is way more complex than simply post here "all healz must deeps" without any context. Specially where healers are context-driven jobs.

But as it has been mentioned already, bad tanking and DPS aren't defended the same way as bad healing is, the other problem is the fact that an idling healer is a more prominent problem than "Ice Mages" in that while these Ice Mages are doing a whole lot wrong, they are still doing something (little damage > no damage), while a healer who is either idling or healing a player who's health is already full is doing absolutely nothing, and it's plain to see.

Again:

The healer won't be granted that he will always have 10 free seconds without any noticeable incoming damage. That time happens, but it is more frequent if the party is good. But if the party is bad? That happens often, you know. And you - and every other defensor of healer DPS - are here generalizing as all healers that don't dps are because they're lazy, they're bad and they are always doing nothing on downtime. Like if every party was good, full of great players. But that healer.

As I said, the better the party healer is in, the greater is his his DPS window. And likewise, the worse is his party, the smaller is this window. And that can reach near-zero.