Security ... think about it!
This is a very important point to tease out. A professional, all-volunteer military is more than just a selective (and self-selecting) group. Service comes with a social compact: Deployed troops should have access to first-rate medical care; they will receive psychological screening; and their families should have access to a support network. While those ideals do not always translate to practice, the American public generally will spare no expense to make sure its volunteer military is properly trained, extremely well equipped and well protected. Deploying as a unit, rather than sending individual replacements, also bolsters morale. All of those factors — the support network, the screening, the good care, the willingness to be there – help cut down on tensions which could eventually lead to fratricide.