Multiple trustees is the main reason. Wife and I are both on it as trustees. If we die my daughter takes over. No clue what happens if we die before she is 21.
While it is unlikely the ATF would come by the house while my wife was there and I wasn’t. Since she has access to the safe they could press the issue of her having possession of an NFA item. Not likely but for $160 for a trust, why not just remove that risk altogether.