You are assuming the Han are a single homogenous people. They are different in the North, South, and West. The people of the coast are different from those of the interior. They have different local languages and local customs. They are large pockets of various religions around the country. China hasn't been unified more or less throughout history. It has pulsed in cycles that ebb and flow. At times it has been unified but just as often it has been broken into warring factions.
The + mark denotes a time of union covering all or most of present day China. Keep in mind that the dynasty only covered the area shown at its maximum and was less during its growth and decline. Total time since the Han for greater China is 1502 years. Total time since the Han for lesser China is 615 years. More than forty percent of China's time was spent as a smaller China. That doesn't include losses during each dynasty during transitions.
China's great strength lies in it's long common history. Until the Qing dynasty the mountains, desert, rivers and sea geographically protected it from outside influences.
2100b-1800b.....Xia Dynasty.....300yrs
1800b-1700b..................no dynasty.....100
1700b-1027b.....Shang Dynasty.....673
1027b-_221b.....Zhou Dynasty.....806
_221b-_207b.....Qin Dynasty.....14
_206b- _220..+..Han Dynasty.....426
_220 - _265.....Three Kingdoms.....45
_265 - _420..+..Chin Dynasty.....155
_420 - _618.................no dynasty.....198
_618 - _907..+..T'ang Dynasty.....289
_907 - _960.................no dynasty.....53
_960 - 1279.....Song Dynasty.....319
1279 - 1368..+..Yuan Dynasty, a part of the Mongol Empire.....89
1368 - 1644..+..Ming Dynasty.....276
1644 - 1911..+..Qing Dynasty.....267
