User blog comment:The Inceptionist/A Multiplayer DLC Idea/@comment-1780148-20120730230947/@comment-1900300-20120731053942

The codex states that Collectors where present at the invasion of Palaven, so there are some remaining forces still alive and in use by the Reapers, so their use in ME3 would not have been deal baeaking. So the most obvious excuse would be laziness. Bioware had limited the enemies to Geth, Cerberus or Reaper forces. Adding the Collectors to the Reaper forces would render Cannibals redundent and giving them a force of their own would require designing alternate forces to alredy existing Collectors and giving explanations. So yeah, you cant blame them for not adding them