Using Fake Artillery in the War

On disguising things in the war. One tactic that was used was to hide real artillery under fake trucks, and also to hidereal artillery under structures which looked like badly camouflaged dummy structures. They manufactured lots of paper machet heads, and they put em on a stick. So if you were in a trench it'd be very dangerous, obviously, if snipers were a shooting at the trench. Why didn't they always do that? I why did anyone die in the trenches?