Cadets leave after two years because your military commitment doesn't start until your junior year at west point and those cadets decided west point isn't for them, or they are LDS and are leaving after two years to go serve a mission and will return when they finish.
I would like to discuss what it takes to get into the USMA with you. I am trying to get in and would like some of my questions answered (please) if you are willing to help my email is crow5839@franklinshools.org