The tank is expected to lead the way, set the pace, etc. so people tend to assume it's something you don't even attempt until you're a relatively experienced veteran. Some people are never really comfortable in that position and with that responsibility. Once you get used to it though it's pretty easy.
I think functionally DPS is actually harder than both healing and tanking. It has the most difficult rotation and very little slack for making mistakes with mechanics; it feels like more often than not healers will just let DPS die if they make a mistake, and if they screw up a mechanic they die FAST.
As a tank if I screw up a mechanic I have mitigation I can pop, self-heals and the healer is always prioritizing me. Pretty forgiving. And healers are just mechanically the easiest classes in this game; literal 1 button DPS rotation and basically deciding between single target or party wide healing based on what attack is coming. Not exactly overwhelming.
