The Allies made short work of the German bunkers during D-Day.
Also, the vast majority of them are the size of a small igloo barely big enough for two, with foot thick reinforced concrete - and they’re everywhere in an onion like structure.
The idea then is that the infantry man retreats to the next layer (some 100 yards or less).
Sure an aircraft an destroy anyone of them. But there are more bunkers than any Air Force can blow up.
Meanwhile the invader has hundreds of kilometers of layers of bunkers to walk through.
Most of the igloos are for rifle men. Every third igloo is open in the back - that’s the RPG guy for the tanks.
Those big bunkers in the photos are quite rare.
Finally, the “commissioning” of the igloo bunker was done by having their designer sit in one of them while the artillary hit it - they’re tough little things.
It would be such a little thing to dump the earth they excavated when building the bunker back on top of the thing and spread it around so grass grew etc, hiding the damn thing.
Imagine being a soldier sat in one. It would give the feeling of 'please shoot at me!' rather than the feeling of security, surely.
For accounts of this by survivors,
"D Day Through German Eyes" by Eckhertz
https://emedicine.medscape.com/article/833665-clinical
"Increased ambient environmental temperature from burning napalm has been known to cause the deaths of individuals in raid shelters as a result of radiant heat and dehydration. This was a frequent cause of death in the bombing raids carried out over Hamburg, Germany, during World War II."
But the air force dropped a very small conventional bomb, sucked all the oxygen out of the air, and put out the fire.
It was in the news a bit. A quick google finds https://www.thelocal.se/20180725/watch-swedish-fighter-jets-...
Easier said than done, and I don't think camouflaging them was ever a priority. Even today, you can see the excavated material spilling down the hill in front of large bunkers 70+ years later, which makes them fairly easy to locate: https://www.cios.org.je/assets/images/other/noirmont-headlan... (this location is also easy to spot in wartime RAF reconnaissance images that I've seen)
"Its roof (3.40 to 7.0 metres (11.15 to 22.97 ft) of steel-reinforced concrete)"
"Because it seemed impossible to destroy Valentin by bombing it, the decision was made to destroy it by blasting. This idea was later abandoned because the blasting would have caused severe damage to the nearby villages of Rekum and Farge including the power-station in Farge. In 1960 the bunker was taken over by the German Navy, for use as a storage depot. "[1]
[1]https://en.wikipedia.org/wiki/Valentin_submarine_pens#Post_w...
"It was the most powerful non-atomic aerial bomb used in combat until 2017"
"The Lancasters attacking Valentin each carried a single large earthquake bomb – seven carried the 5 ton 'Tallboy', thirteen carried the 10 ton 'Grand Slam'. Two 'Grand Slam's hit the target and penetrated about half-way through the 15-foot (4.6 m) thick ferrous concrete roof before exploding. The explosions blew large holes in the remaining thickness of the roof and brought down around 1,000 tons of debris into the chamber below.[8] Workers who were inside the bunker at the time survived, as the bombs did not penetrate the roof before detonating.[8]"
https://en.wikipedia.org/wiki/File:U-Boat_Pen_Grand_Slammed....
https://en.wikipedia.org/wiki/File:Bremen-Farge,_Royal_Air_F...
A bunker cannot maneuver or adapt under fire, meaning it's very vulnerable to modern warfare techniques. Mostly it just winds up as a trap for the unfortunates in it.