Hollywood has to take shortcuts to tell an epic war story, or we'd never get films into the theaters. Complex stories are simplified, timeframes are shortened, and (at least before CGI) sets are as small as they could get away with. The vast majority of Americans who've never been in a battle easily get the wrong idea about what war is like if what they know comes from watching movies. For example, the trenches of World War I are shown to illustrate the horrors of that war: tiny, muddy, crowded hideaways where soldiers are constantly under fire. As they were in use for years, those trenches were actually much more sophisticated.
In fact, the complexity of the trenches was designed to counter another favored stereotype of the First World War -- that soldiers were seen as easily replaceable machine-gun fodder. The trenches were cut as zig-zags so that, should an enemy force breach the lines, all the troops weren't just standing in a straight headshot-able line from the North Sea to the Alps. Also, the walls were paneled with wood and the parapets reinforced with a ceaseless line of sandbags, and there were large medical stations installed throughout, because even these generals took care not to just let entire regiments die from gangrene and patience.
That's just the beginning of the complexity of World War I trenches. They were sometimes even connected underground to larger staging areas. Read about them and four other parts of war that aren't exactly as Hollywood portrays them at Cracked.