Also, why do people think that the enemies of the Ancients had to be advanced ? I know that there's no cannon evidence, so we can only speculate... But still, I find it hard to believe that EVERY civilization they encountered had warships or even was space fairing.
It is highly probable, that many of the civilisations were at different levels of development, so many Ancient conquest could be similar to the modern Terrans nuking a medieval society. They wouldn't stand the slightest chance. Races at that stage could even simply surrender, terrified by the view of divine creatures coming from the sky.
If that would be the case, it becomes fairly obvious why the Ancients would become so arrogant. Without or with only a small amount of opponents capable of fighting them back, the Ancients would be slowed down in improving their tech. They probably focused more on ways of efficient expanding their empire.
EDIT: Okey, my bad, they were colonizing systems for thousand of years. Poor memory I guess. Sorry.