Because it's a word that got integrated into pop culture that people tend to use for everything and anything remotely emotional.
The thing, people tend to be emotional, because....we have emotions. Emo was used in pop culture nowadays to describe the overly whinny punk bands that emerged, the ones that wore all the dark make up and the little teenagers copied and made it into a living walking joke.
Personally, I dont see any of Gackts song's that way, or really TONS of other bands. If Gackt is emo, then so is literally every other band in existence since most people tend to sing about love and loss.
Either way, it's a highly irritating word.