You beat me to it.
Also, by the time of Julius Caesar, the Celts were in the process of having their tail repeatedly kicked by Germans.
JC moved in and killed something 1/3 like of all the (Celtic) Gauls, selling another 1/3 of them into slavery.
As it happened, the Romans conquered the Celts just before the Germans would have.
Other Celtic groups (Irish, Welsh, Scots) generally have throughout history been far too busy fighting and stabbing each other in the back to form themselves into effective military forces against outsiders. Celtic soldiers have been most effective when conquered and incorporated into the armies of their conquerors, when, to be fair, they have been amazingly effective.