Because tanks and DPS are most of the time doing something to contribute towards a common goal, as in pressing a button for damage or damage mitigation every 2.5 seconds (GCD), while some healers are either not pressing any buttons to contribute towards said goal, or are pressing the wrong button (healing people already at full health) to the benefit of no one.
I can't stress enough how good this guide is, and how well it explains what you are supposed to do as a healer and why.
Are you prepared to suffer?