SG-1 has been on a mission for five years.
The string of Generals (and a few Doctors) that have been put in command of the SGC have tried to have them declared MIA, their team designation recycled. One had even tried to have them declared AWOL when that had failed. No SGC veteran would take a place on SG-1. SG-1 is a legend, the only team left from the beginning of the program- and the only one that hasn't permanently lost a member, despite all of them dying more than once. They've all heard the stories about SG-1, from their first day in SGC orientation. In many ways, the story of the SGC is the story of SG-1. Refusing to take a spot on SG-1 has gotten more than one person dismissed from the SGC, but they agree it was worth it, despite never being able to go through the Gate again.
Having been refused by the veterans, a wise person would have given up, but most of the commanders were not wise people. They wanted to make their marks on the SGC, and for many of them that meant being the one to put a new SG-1 in place. Unable to use SGC veterans, they turned to new recruits.
There is a reason new recruits have a long period of training before being placed with an established team, rather than the shorter training period there was elsewhere or forming new teams out of the new recruits. The casualty rate is highest in the first six months, regardless of prior experience elsewhere, and goes up exponentially for each member of a team that is new. The casualty rate at the SGC isn't exactly low in the first place, even without adding risk factors that are carefully minimized most of the time. Pulling a stunt like that is a death knell for the commander who does it. The veterans might follow the regulations to the letter (or at least as close to the letter as things ever got in the SGC), but they'll never respect them again. Even trying to replace SG-1 is a sign that they aren't a true part of the SGC.
The "new SG-1"s are often the best and the brightest, trained to be diplomatic and friendly- after all, SG-1 is the flagship team. They have to put their best foot forward. Under other circumstances, they would have been accepted with open arms. The SGC can always use the best of the best, and everybody knows it. But the veterans don't accept them. Oh, nobody does anything overt. There's never any harassment or bullying, or anything that could be complained about without looking like an idiot. But there's more than one way to make things uncomfortable. Everybody's polite enough, but that's as far as it goes. The veterans save their lives, but there's no camaraderie. There are no invitations to join any of the activities. They make no friends. Conversations have a tendency to stop when they enter a room. There never seems to be any blue Jell-O left. And nobody refers to them as SG-1 except the commander. They're smart people; it doesn't take them long to figure it out. The commanders are a different case, but they never seemed to last long anyway. The veterans like to think that it's because none of the commanders has truly been part of the SGC since General Hammond. They've all started out as the commander, rather than as a member of a team going through the Gate.
SG-1 was a part of the SGC from the beginning, surviving for eleven years with unchanged membership (except for the year when Dr. Jackson was Ascended) before they went on their latest mission. Somewhere in a computer, they're probably listed as missing in action, but no SGC veteran believes they're dead. Sure, they're five years late for their check-in, but they're still alive somewhere; they're SG-1, after all.
