I'm assuming because before the Academy reopened at Starfleet Headquarters and given the nature of the Alpha Quadrant, post-burn cadets most likely had to get practical, real world experience as quickly as possible. This most likely meant that they had to be trained on starships or there was a dedicated starship that served as a mobile training facility. It's probable that this method worked well and they decided to keep it when the academy reopened on Earth. Which is why they havea Captain serving as both the chancellor and commanding officer.That makes no sense to me. Why would the dean head off on these missions? What about all the students left on earth and all the Academy stuff happening off the ship. What about all the bureaucracy that comes with the job of being a Dean that conflicts with the day to day operations of commanding a ship. Trek IMO has gotten this right in the past with some of the Admirals that would come aboard the Enterprise. They would be in charge of the mission but Picard would remain in charge of the ship.