Why has the dream of many medieval monarchs to create an empire as powerful as the ancient Roman

Why has the dream of many medieval monarchs to create an empire as powerful as the ancient Roman Empire remained a dream? What prevented this dream from coming true?

On the territory of Europe during the Middle Ages, there was a constant struggle for power between states and their rulers. Each tried to capture more, but none of them managed to build something like the Roman Empire. The only one who more or less approached this goal was Charlemagne, but his empire only vaguely resembled the huge Roman Empire, stretching from west to east for thousands of kilometers.