When the Roman Empire collapsed, no-one after Justinian ever attempted to re-unite it ever again.
In Chinese history, even during times of significant upheveal on a similar level to the Migration Era, such as during the Sixteen Kingdoms, Northern and Southern Dynasties and the Five Dynasties Ten Kingdoms eras where numerous ethnically non-Han peoples moved in and lived in China proper, China would always manage to re-unite itself (E.g. The Sui Dynasty would re-unite China after the Sixteen Kingdoms and North-South Dynasties era and the Song Dynasty would re-unite China after the Five Dynasties Ten Kingdoms era).
Why was this the case in China and not the case in Europe?
I will give you answers about China, because explaining why something did happen is easier than why something didn't.
/u/necksbetrim, /u/touchme5eva, and /u/Dongzhou3kingdoms have previously addressed China is famous for going through a cycle of strong empires and fracturing and civil wars. But did people during times of small kingdoms see it as a “civil war” or as a war between separate states? Was there a desire or expectation that they’d all be united again eventually?
(see below)