You mean the Dreadnoughts? They haven't been seen probably because, with the exception of Azys Lla and a few Castrum cells, The Garlens don't have a strong military presence in Eorzea at the moment.

They were used in The Battle of Silver Tear Skies and the invasion of the Black Shroud (Likly the capturing of the Castrum sights too), but other than that they don't seem to be used as much as "Conventional Weapons" like the other Magitek. The Colossus sees very limited use as well and seems to be a special Heavy Infantry that's only deployed to high risk areas. (Like Castrums and The Agrius)