I have noticed this as well.  In one game (Expert difficulty), 5 of the 7 AI on a large map quickly raised three of the towers for the Spell of Making victory- and then failed to complete the only tower with resource requirements for the next 100-something turns or so as I gradually wiped them off the map.  They had plenty of resources in their respective borders, but the AI was usually very low on metal and almost never had any crystal.  The metal deficit may have been explained by the AI quickly spending it on new troops, but very few if any of the cities I captured had improved their crystal resources.  I never faced any AI troops armed with magic staves or enchanted weapons / armor.  On one hand, I'm glad that the AI wasn't able to finish a spell victory before I even had a chance to explore the map, but at the same time it I fought far too many late-game AI armies where most troops were clad solely in leather armor, wielding maces.