In a way, the basic military logic that it's better to flank the enemy than attack head on is WHY the trenches extended from the North Sea to Switzerland. Commanders from each side kept attempting to flank one another in the traditional way but innovations in logistics meant that WWI was the first war fought where the armies numbered in the millions of men, so when they tried to maneuver around the side of an army like they'd been trained they just found more army.

They both kept going further and further with attempted flank after attempted flank until they both hit the metaphorical walls and then nobody really knew what do to next because they found themselves in a completely unprecedented military situation. Military theorists eventually figured out how to punch through a line with narrow concentration of force and flank that way, but it took bloody trial and error.