- Out of habit. In most MMOs (and I'm not talking about that one actually) "battle priest" is kind of a curse word. Even if a healer can dish some damage his teammates will encourage him not to do so having no confidence in the healer's ability to do damage and keep them alive. So most of the experienced healers won't damage if they are not with friends or until they see that tank won't die trying to bite too much.

- It's not their role aka I don't want it. Holy trinity, of course, as it was said before. Playing as a party means everyone do their role first and foremost. It also can have a root in real life and not wanting to do others job. You know, even if you can do everything you need to know how to delegate tasks to others or you would be overwhelmed while others are slacking around.

- Playing safe. You may want tanks and healers to dps but you'd rage if they don't tank or heal properly. So to avoid being called on and losing time with the wipe healers just heal. Basically, healer doesn't have confidence in his teammates (always the DF case) and/or in his knowledge of the fight and want run to go smoothly.

- Their damage is too good. Just because I had this situation while leveling:) I used to get mobs attention if I have time to get to the "Bane" as SCH. And running from mobs while tank try to catch them (if he even try) isn't fun at all.

- They have bad connection. Or had it before in any game. It's also a reason for full heal spell for every little scratch. Healers usually look onto health bars of the party, but bars they see aren't the actual hp of the teammates. There is some latency after receiving damage, changing its visual representation on bars and communicating this change to other players. So even if they react as soon as they see hp dropped beyond certain point they've lost couple of seconds + seconds to switch stance and target which result in 1-2 missed regular heals. How relevant these heals are can be different in different cases. Btw, even if healers don't think about this as a connection issue in most cases "they are afraid they won't have time to switch off/smb dies while they switching off" connection is a part of it. With bad connection hp drops may be surprising and fast.

- Bad examples. Cases when healer is too involved in dps to heal are quite noticable and nobody is happy in those runs. Seeing how tank's hp drops to 1-3% because healer turn off cleric too late and went to use regular heals is the prime example not to try switching into dps at all.

- They actually have things to do. Character and player isn't of the same body and mind. Player can do such mundane tasks as watching hp bars to heal damage asap, planning where to go, looking on "out of range" messages or placement of party members, boss attacks, re-reading guide or explaining in private chat fight to the newbie tank/dps they entered with. Usually cause of it lack of experience. Or he can watch kittens on Youtube and just slack off because he's sure his team also doesn't go full strength in this DF run.