Seems like a dumb question, but here it is: at least in the western front, trenches were heavily defended, and 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?

ps: I understand that breaking the line of defense of the ennemy and conquer their land grant you victory, but it's not the only way: U.S.A. lost Vietnam War, without beiing conquered. They withdraw because they endure too much losses.