Because it means the game is only a number game and that you re not doing anything relevant gameplay wise, in term of impact on the course of an encounter. If the game was well designed on a gameplay standpoint you wouldn't need parsers to tell if someone wasn't doing his job properly. As an easy example coming from FFXI, if to deal damage you needed 2 melee to cast 2 abilities in succession to perform a skillchain then a magic user to perform a magic burst, you'd immediately see who would messing up if something went wrong (as it is a matter of coordination).
It's also the reason why nobody care about PVP in FFXIV, be it players or watchers.
It really is though, I'm not playing FFXIV anymore but every single player of my FC (who raided in savage when it was really hard at the beginning of HW) really did need parsers to improve themselves and complete the floors (as the DPS checks were insanely tights).