There is a fundamental tension that the service academy and full ROTC scholarship selection process. These processes are designed to choose the best candidates they can attract for obvious reasons. The tension comes from the fact that those who earn appointments or full scholarships are positioned in life to take advantage of multiple opportunities.
If someone has the standard resume that these cadets due, they are more likely to succeed than the general population. If they continue their progress, earn degrees in engineering or mathematics and perform well, they will have significant opportunities available to them outside of the military when their commitment is complete. If they are in this group of high performing people, they likely have competitive drive and ambition.
Take a high performing, ambitious, driven individual and then put them on a rigid promotion schedule that most likely will have them paid and promoted like their counterparts who underperform them and you create motivation to consider another career. I have never known anyone to get promoted to Captain below the zone. Few can get to Major below the zone. That puts these folks on the same promotion schedule as everyone else for likely the first 10 to 12 years.
Most of the people in this group understand going in that they are serving their country, performing a fulfilling mission, are prepared to move every few years, and accept a loss of some freedom in life choices. They know these sacrifices, they discuss these with others and prepare for them when possible so they can meet these challenges head on. However, the sacrifice of opportunity and advancement is not addressed.
The system is designed to foster competitiveness. What slot you get can depend on your class rank. Selection to special schools will be based on performance. The cadets are trained for four years to be the best. Then when on Active Duty, the system changes a bit. Sure, high performers may still get access to certain career fields and assignments over others, but they will likely still get promoted like everyone else in the first half of their career. Add to this, the possibility that they work for someone who isn't exceptional and their motivation can drop significantly.
To be completely fair, this is my perspective because this is how I and my friends felt so it is certainly biased. My friends and I were all Academy or Scholarship ROTC. One left, got an MBA and went to work for an Investment bank. One left, went to Stanford Law and then a VC fund. A third, left got a MS Comp Sci and went to work for Google. The fourth, separated went into industry and eventually became an officer of the company. A fifth stayed and retired as an O-5. All of us were motivated. All of us believed in what we were doing. None of us left for the money. We left because of the bureaucracy, slow advancement and lack of a challenge. We believed we could do more than we were given the chance to do.
I readily admit that a portion of that sentiment comes from the naivety of youth. We were young and thought more of ourselves than we should have. Yes we went on to be successful, but still our youthful arrogance was not fully justified. But, it was partially justified. And when you take a group of young people who are bright, driven and ambitious; train them to always seek to be #1; tell them they are the best the country has to offer; fill them with inspirational quotes about challenging yourself and becoming the best you that you can be, you are creating people who want advancement and challenge and who expectations of their leaders. No doubt, others have been through the same system and were blessed with great bosses and assignments that challenged them, or at least had enough that could be described that way so that they could weather the bad boss/assignment.
The first and second job assignments for a young officer are crucial to their development and mindset. If you get a lackluster organization and/or boss like my friends and I did, you sour quickly on the military. As a young man/woman, you lack the perspective of age and experience and instead, project those first few years of dull work and uninspiring leaders into the future so they look for another opportunity. Those who excelled in college or Academy have plenty of opportunity outside the military so many of us pursue that.
Extending the commitment for pilots makes sense, I was USAF so we have been used to that. For other officers though, extending the commitment by one year, is probably not a huge deal. More than that though and you will hurt your chances of getting the best. An 18 year committing to the next 9 years of their life is a lot (4 college/academy + 5 active duty) and is a challenge. My son wants to be a pilot in USAF so he is looking at 4 + 10 to 12 years locked up. The next 14 to 16 years of his life? That's along time for an 18y old. One of the things that helped him join USAFA is that he can pivot during USAFA and go non-rated and only have a 5 year commitment. Still young enough to go to grad school and pursue another career if he chooses, but still has the option to go rated if he wants to.
A better way to increase retention is to focus more on the first two assignments. Make sure the officers in charge of 2Lt and 1Lt are some of your best.