First of all, Haig had planned the Battle of the Somme to be one of the most important of the many battles of World War 1. He had planned it to obliterate the German Line, and clear a path for the British to invade Germany and win the war. He even admitted this in the lead up to the battle. This meant that he was clueless of what could and what eventually did happen.
Secondly, he was very over-confident. Commanders were told, by him, that the 7 day heavy artillery bombardment would kill all of the German defence. So there was no need to run, why run when you could walk? The bombardment turned out to be very unsuccessful; it didn’t manage to cut the barbed wire correctly. Furthermore, the Germans who had just hidden underground all that time simply returned to their trenches, which were pretty much in the same state as they had been left, and shot down the British who were advancing at walking pace. This showed that not only was Haig over-confident, but he was also prepared to stand-by as people quite literally walked to their deaths.
Thirdly, despite the huge catastrophe on the first day