Part of the reason is that Chinese city walls often had secondary uses that the Europeans didn’t have that made large earthworks more viable than other types of walls.

Arguably the most important secondary function for city walls was their function as levees and dikes to counter flooding, which were a frequent and dangerous occurrence among the Yellow River and the Yangtze River. This function is most evident in cities with incomplete walls near these rivers, such as Chungmou, where often the only part of the city wall completed were the ones facing the river. Levees need to be several meters thick and relatively tall to hold back large amounts of water during a flood, which likely contributed to the use of thicker earthworks as city walls. As far as I can tell, few Roman or Medieval fortifications, if any, also functioned as flood defense, while flood defense was a key priority for many city walls along the major Chinese rivers.

Another aspect was the employment of wide moats. Nanjing and Suzhou had 80-meter wide moats, and Beijing and Taiyuan had 30 meter wide moats. These excavations generated a lot of earth, which could be used to simultaneously build the city walls. While the use of excavated earth to build fortifications was also known to Europe (for instance, motte-and-bailey constructions and ringworks), contemporary defense reasons and prestige reasons resulted in wood and earth castles being replaced with stone castles.

There was also the cost. City walls were one of the first things to be constructed in a new Chinese settlement, and they often enclosed a vast area of space to contain artificial bodies of water, agricultural land, and salt lakes alongside housing. As a result, these walls had to cover an area larger than European city walls for the same population. Then as cities grew larger than their walls, satellite walled cities or an outer wall also had to be built to contain the growing population. That lent itself more to earthworks, which were easier to build and gather materials for (especially with the simultaneous construction of a moat or canals).

Sources:

Chang, Sen-Dou. “Some Observations on the Morphology of Chinese Walled Cities.” Annals of the Association of American Geographers 60, no. 1 (1970): 63–91.

Liddiard, Robert. Castles in Context: Power, Symbolism and Landscape, 1066 to 1500. Oxbow Books, 2012. https://doi.org/10.2307/j.ctv138wt8d.

Kenyon, John. Medieval Fortifications. Leicester University Press, 1990.

To add to the excellent points about flood control that u/LothernSeaguardOne makes, another suggestion that Andrade discusses is that geological stability was another important factor. Specifically, Western and Northern Europe are relatively much less prone to earthquakes than Japan, Korea or even much of mainland China. That changes the cost/benefit calculus of certain methods of construction - and it is that cost/benefit calculation that I want to elaborate on here.

Rammed earth wall construction is incredibly resilient to earthquake activity in a way that even a thick vertical stone wall is not, but it is very labour intensive to implement on a large scale. That means that it may well be worth doing rammed earth fortification if it means you don't have to rebuild the walls regularly, and if you have a centralised state with a dense population that can coordinate very large unskilled labour forces. Another 'cost' of earthquake-resilient rammed earth fortification is that it generally results in a sloping wall rather than a vertical one - and sloping walls are relatively easier to escalade / climb. They thus need to be somewhat better manned to prevent being taken quickly by storming, which imposes an additional passive cost (paying more soldiers) to maintaining your fortification.

Now, note that there are two main types of fortification, and their relative incidence was different in medieval north-western Europe and in East Asia. Castles were essentially fortified manor houses which, militarily, acted as bases from which to interdict an enemy army's ability to forage in a region and thus move through it, and culturally, acted as seats of power for landed nobility to administer their territories. Those nobles had limited access to surplus labour, as population densities in the regions that were conveniently controlled by castles were usually quite low, and the costs of manning these castles needed to be kept in check. In medieval Christendom, and especially in Northern and Western Europe, castles were particularly common, partly because of the lack of the systems of highly centralised administration that were common in China and in the Classical or Early Modern periods. If you are a European noble with a castle, who has a limited unskilled labour supply (due to both lower population density and less sophisticated administrative systems), and few natural disasters that would make rammed earth construction much more efficient over the long term, it makes good sense to go with vertical stone walls wherever possible.

The other kind of fortification is a fortified town or city. These are much more common than castles in societies with powerful, centralised states - such as East Asia and in the Classical Mediterranean. Fortified towns and cities can have more labour available, and manning the fortifications is a bit easier, so rammed earth is perhaps a bit more feasible - but the cost/benefit is still complicated. Does the city have a lot of skilled stonemasons, for whom building and maintaining a stone wall is relatively straightforward, and may even be a source of civic pride? Is there the physical space available for the larger footprint of a rammed earth wall? Does the city need a wall very quickly, as Athens did after Xerxes' invasion (they used salvaged stone from their ruined city to build new walls very fast indeed)? These decisions are complex, and people's reasoning varies from place to place and will be shaped both by cultural convention and by a wide range of practical and political factors.

Thus, a combination of environmental factors (the prevalence of earthquakes and floods), political and economic factors (structures of labour supply, population density, and the relative costs of construction vs maintenance and rebuilding), and cultural factors (the prestige of stone fortifications and the symbolic role of the castle / walls) shaped different decisions in different locations.

u/LothernSeaguard gives a fantastic response from a materialist perspective, but there’s also a strong answer to this from a sociocultural, or if you want to call it that, social constructionist perspective.

The Chinese empires were first and foremost, some of the most decentralized to ever exist. By all means, biopower was concentrated, but most biopower wasn’t actually exploited. The countryside was generally strongly autonomous and methods of administration varied strongly by region. At a glance this seems counterintuitive - what should we make of immense armies and monumental building projects? But these were largely scattered occurrences. Neither the Great Wall nor the Grand Canal were being built every year of Chinese history, and war obviously wasn’t a constant either.^(1)

This is related to how Chinese sovereignty is different from European sovereignty: aside from those rare circumstances, the social contract was usually some variant of “if you give us money or labor and tell us we’re superior, we’ll leave you alone” (the money/labor part was negotiable).^(2) So if one were a foreign invader looking in, what really matters is the cities; the places where administration stopped being so localized and one could simply grab the resources and go, or where megaprojects and military campaigns gathered their recruits. And all empires are, on some level, foreign invaders.But let’s look back at those colossal Chinese walls.

Fortifications were above all, megaprojects. As LothernSeaguard pointed out, they were usually built first, before anything else in a city, and as part of the same process as digging moats. European fortifications, at least pre-Mongols, were usually spread out far more and designed to improve the surrender conditions for each individual town, because minor centers were important under feudal systems and peasants could be called to repair palisades every year.^(3) In contrast, the situation in China was a state apparatus concentrated in cities, where far more labor could be exploited but only once or twice a generation. So it’s only natural to build fewer, higher quality but far more expensive defenses, isn’t it?

^(1 Also contributing to this narrative, I think, is just how much paper and ink was involved in the bureaucracies. But bureaucracies imply information, not power. In practice there was a bewildering diversity of disjointed regional titles and terms which often meant different things in different places, which meant that locals generally had an advantage over faraway administrators in navigating this system (for example, a clan in Fuqing bothered by state conscriptors got away by tracking down and retroactively adopting a long-lost cousin already serving in the army).)

^(There’s also the element of projecting modern China back in time, but that’s another story.)

^(2 What foreign scholars call “tributaries” were just parts of the empire that did negotiate that clause. Naturally most of these polities were also a lot laxer with the superiority rhetoric when China had its back turned.)

^(3 This is shorthand. Please don’t flay me, medievalists.)