How did trench warfare make offensives long and deadly?

Trench Warfare is a technique where belligerents create earthworks (often trenches) that protect troops from small-arms fire and some protection against artillery. World War I is known for that, and so European east theater became riddled with networks of trenches. A trench well protected by other subsequent trenches and artillery makes offensives costly and deadly to the side of attacker, and so often the conflict becomes some sort of a stalemate.