Seems like a dumb question, but here it is: at least on the western front, trenches were heavily defended. Any charge was just suicidal, with heavy losses for the attacker and much smaller losses for the defender. As there is a big strategic advantage to be the defender and not the attacker, why bother launch an attack?

I understand that breaking the line of defense of the enemy and conquer their land grants you victory, but it's not the only way: the USA lost the Vietnam War, without being conquered. They withdrew because they endured too much of a loss.