according to the book, what important reason does the state have for not ordering is citizens to build pyramids or temples instead of weapons if the aim is just to keep them busy?

From the text:

"In principle it would be quite simple to waste the surplus labour of the world by building temples and pyramids, by digging holes and filling them up again, or even by producing vast quantities of goods and then setting fire to them. But this would provide only the economic and not the emotional basis for a hierarchical society. What is concerned here is not the morale of masses, whose attitude is unimportant so long as they are kept steadily at work, but the morale of the Party itself. "

Sorry, I left out the second and most important part of the text, which explains that the party wants its population to be consistently prepared for war.

"What is concerned here is not the morale of masses, whose attitude is unimportant so long as they are kept steadily at work, but the morale of the Party itself. Even the humblest Party member is expected to be competent, industrious, and even intelligent within narrow limits, but it is also necessary that he should be a credulous and ignorant fanatic whose prevailing moods are fear, hatred, adulation, and orgiastic triumph. In other words it is necessary that he should have the mentality appropriate to a state of war."