Because the healing requirement in this game is so low. It's not about what you are doing, it is about making sure that you are always casting something useful. If you are 100% healing then you are overhealing about 75% of the time, which is completely useless to the party. People don't care about a healers DPS numbers (with exception to DPS checks in high end raiding, I.e gordias), but they do care how much time you spend doing useless things like idling and overhealing.
IMO, anyone who says "Why do I have to DPS? Is my healing not good enough?" hasn't properly played healer, because they should understand how low the healing requirement is (I'm a noob healer and even back when it was current content, the first boss of antitower could be done on SCH without me pressing any buttons other than dodging mechanics).