Partially, this is due to the increasing detachment of the U.S. Military from the General Public since the 1970s; when large scale draft forces were retired in favor of the all-volunteer force. Since then, we have had no war or disaster large enough to require the bringing back of the draft; and thus today's science fiction writers are not as experienced as many of those in the golden age of science fiction, where many authors had World War II experiences to draw upon.
Trope 1: Super-Tough Training Makes Super-Men!
I'm sure you've read at least one story that goes into detail about how brutal and how tough the training is for such unit; and that out of 50 men who enter, 35 are killed, or washed out of the unit (or some such number).
Naturally, this training makes the survivors into nigh unkillable supermen.
In real life, it doesn't work this way. Each person that dies or is washed out of training is a person whose skills you have lost forever. This is especially important if the training relies on a lot of very realistic live fire training. Modern warfare is now essentially a game of chance; the person who scored a 98 on the rifle proficiency test can get killed just as easily by a stray bullet as the guy who barely passed the aforementioned test.
Trope 1a: But what about Special Forces Training?
The reason Special Forces units make the training so tough is for two main reasons:
- There simply aren't enough slots in the unit for all the people who apply -- in 2001, before the huge expansion of special forces in the US Army, the Rangers numbered about 2,000, and the Green Berets about 5,000. So in order to keep the special forces units within their existing authorized strength, a lot of people must be weeded out. Making the training tough gives you a credible excuse to fail people.
- Because Special Forces missions usually go pear shaped disturbingly often, you need people who are absolutely dedicated to the mission, who will try to finish it no matter what the cost (mentally or physically).
This does not make them better soldiers. What it does is it makes sure that the right type of personalities get into the unit.
The chances of success in a typical SF mission are already bad enough; and in many missions, there are absolutes tacked onto the mission -- e.g. you must assault that airfield at a specific time, despite any problems you might suffer along the way, such as 25% of your team disappearing during the initial swim from a submarine to shore due to hypothermia.
When a mission becomes completely unhinged, and it turns from achieving the mission objective to simple survival, you need people who will keep going no matter what the cost.
In a normal military unit, you can detach people to carry the wounded or order the unit to slow down from double time to give people a chance to catch their breath.
This is not possible for SEAL TEAM FIFTY SEVEN who are exfiltrating after they successfully attacked an Outer Loonystani research facility and destroyed the Super Bubonic Plague that they were working on (along with the researchers); and are now being mercilessly hunted by the Outer Loonystani military, special secret police, and Young Pioneers. People who slow down in such circumstances have to be left behind, and you have to operate on little or no rest at all until you're safely across the border or something.
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That should do for a start. I will expound a bit more on Special Forces; most of the problems i've encountered seem to trend from such elite units, whether it's Michael Z Williamson, John Ringo, or even John Scalzi.
