Being born ready could be convenient because it could be like a natural stasis for them while they are on the transport ship, then they are born a few days before they reach their destination. They would have a dreamlike state once they are mature enough inside their "eggs" (probably not literal...but only probably) so that they already have some "real world" simulation experience to work with. They are also vaguely aware of the outside, so if one started to attack the eggs then they'd know to hatch manually, even if it's a bit too early. They grow at an enhanced speed inside the egg and can be considered ready anywhere between appearing to be 12 and 16 years old, the sweet spot for this form of birth. The elite adult Evvies that start from birth are never transported like this due to the increased danger level in their much longer and drawn out vulnerable state. They stay in the top secret facilities and do training for all of their lives once they are old enough.