User blog comment:Omega natsu2/Chapter 359: Fairies vs. Hell, Predictions./@comment-1359322-20131101065020/@comment-1359322-20131101102015

I don't think they already incorporated all, they still need some Dark Guilds to recruit people after all. If all the Dark Guilds under them are gone, then Tartaros would get very limited mages to work under them.