The Romans weren't any more significantly wealthier than other contemporary Old World civilizations/empires - China, Persia, or India, for example. Conquerors being assimilated is a common theme that runs across most of these civilizations as well. However, monumental buildings that survive invading armies just tend to be dismantled/repurposed by the locals for the building material over time. I suspect that part of the reason for Roman monuments surviving is that the fall of Western Rome lead to an overall decrease in the urban population & subsequent demand for building materials. China, India, & Persia likely saw no such population decreases.