One has to wonder whether the "brass" were utterly devoid of basic common sense, or if they actually had a mandate to "thin the herd" because there was high unemployment among young men or something.
There were several problems:
First, the British (from whom most of our WWI lore comes) had a small but highly-trained professional Army at the outbreak of the war. They had to build a mass army "under fire" (John Keegan's words.) The pre-war professionals had a low opinion of the new troops' ability to do anything complex, so the simplest tactics were adopted -- march in line abreast into the enemy trenches.
Second, an Army is like a battleship, difficult to turn around. Despite any attempts at the top to develop new methods, the old methods persisted in the trenches for a long time.
Third, ranking officers did
not visit the trenches and see for themselves. Sir John French, the first commander of the BEF only went into the trenches to say good bye when he was relieved. He burst into tears, saying, "My God! Did we send men to fight under these conditions?" His successor, Douglas Haig, wouldn't even visit the wounded for fear it would make him "soft."
Fourth, the great fear among the Allies was that Russia would collapse (as actually happened in 1917.) To keep Russia in the war, the western Allies had to continually attack to take some pressure off the Russians. The Germans for their part mostly stayed on the defensive in the west and slaughtered the attacking British and French -- hundreds of thousands of lives were sacrificed for this political aim.
Fifth, there was no good battlefield communication system. Troops would advance laying telephone wire behind them, but the wires would be quickly cut by enemy artillery fire. A radio tranciever in those days would have filled a wagon. Without communications, more sophisticated tactics were much more difficult to execute.
Sixth, back to the battleship analogy -- at the beganning of the war the British placed huge orders for equipment, weapons and munitions, based on their understanding of the need. In prevous wars, they had found highly-mobile field artillery firing shrapnel to be most effective -- so they ordered light guns and lots of shrapnel. What they needed was heavy guns and High Explosive (HE) ammunition. One result of this early mistake was that the lengthy bombardment preceding the Battle of the Somme in 1916 was almost totally ineffective.
