There's only an Achievement for the first raise and I think for 20 raises. People who main healers probably cap pretty quickly. Therefore it probably IS the kindness of strangers and not a desire for Achievements that compels people to raise you and any other players they see lying dead. Other people have answered why healers stand back from a Fate, but to chime in, if you are ranged you don't need to be up close and personal with the Fate boss.