...because right now, a level 4+ Vasari Rebel Titan will basically autowin vs. arbitrarily large AI fleet(s), since they will just let all their ships sit in range of the ability, getting hammered with uncapped AoE while the titan enjoys an endless supply of HP and antimatter. Once you've charged into the middle of the enemy fleet, it's already over.
If they ran their ships away while hitting the titan with bombers, combat would actually pose a threat, because right now, the only danger hostiles pose is the ability to attack the gravity wells where my titan isn't.
