Especially when you're behind enemy lines. You've got a job to do, you're not going to let an injured man hinder your progress.
The best way to make the point is from the Battle of the Black Sea (Black Hawk Down). When Blackburn fell from the Blackhawk and it took six other soldiers to pull him back to the convoy. British troops would have had him treated and the other five soldiers would have stayed part of the battle. He probably wouldn't have been left behind but they wouldn't have bothered with him while the battle was still raging, only a medic would tend to him.
Or when the first Blackhawk went down and they spent hours cutting out the cockpit to bring out, what they found to be, dead bodies. The British would have left them. They would have probably gone to the crash site, picked up any survivors, blown up the helicopter and gone.
In the Falklands those that died there, were buried there. Most of the time, we don't even bother bringing the bodies home. I think they're starting to do it more and more now though.