To understand this, you need to understand the economics of imperialism. New Imperialism in the 19th century was motivated primarily by economic factors. This is not to say that nationalistic and ideological reasons did not exist (these factors created a fair number of disputes that had no basis in economic interests), but that they took a back seat to national and economic interests in the formulation of policy.

Economically, new imperialism's primary benefit to a nation was to provide a favorable "balance of payments' Situation to its government. Every government's currency is demanded both by its own people and by foreigners. The more the nation exports and the less it imports, the more demand for the currency rises in the rest of the word, allowing the government to print more currency and essentially gain free money. Governments offer a product too, and that product is their currency. All economic actions they do, from manipulating interest rates to negotiating trade agreements to even levying taxes, are attempts to control the supply and demand for their currency by either taking currency out of the market or increasing demand for it. Revenue does not come from taxation, but from issuing money. Debt and taxation are simply ways to control the supply of money in the market so the government can print more money.

Because of this, European imperialism was loose. The influence of European governments on their most remote provinces was scant, nad usually involved treaties of subservience with local proxy rulers, such as the Khan of Bukhara in the case of Russia or the Nizam of Hyderabad in the case of Britain. Direct control varied based on national preferences - the French, with more idealistic motives towards their African possessions and a tradition of a well-developed professional bureaucracy were far more direct in their control than the British. The British were perhaps the most indirect, preferring the approach of "Empire on the Cheap" - at one point, Anglo-Egyptian Sudan was controlled by less than 400 military and administrative staff.

However loose imperial control might have been, it was almost beneficial for economic reasons. By controlling market access, Western governments could prohibit local industries competing with their own and force local proxy rulers to allow all Western imports. They could also force these local authorities into contracts with European countries to build infrastructure or provide security.

This objective was largely achieved in China so direct rule became unnecessary. Before the 1840s, China was able to heavily regulate and restrict trade out of the port of Guangzhou, allowing it to manipulate a favorable trade balance with the rest of the world. In response, Britain permitted opium smuggling to China. When smugglers were caught and allegedly mishandled, Britain used this as an excuse to go to war, confident in naval superiority. China was forced to sign treaties allowing foreign access to its ports, allowing foreigners to dump whatever goods they wanted into Chinese markets.

Territorial imaginations of this concept, later termed "spheres of influence", stemmed from a misunderstanding of the motivations of governments and business by non-economic academia and the popular press. It is not uncommon even in modern history textbooks to see China and Persia partitioned into territorial "spheres" by European powers, but the reality was that there was no strict territorial division of economic interests. Powers were able to export to China and get China to pay European companies to construct infrastructure.

However, full conquest of China was not worth the cost for a number of reasons. First, China, despite naval weakness, was a formidable land power. Qing and Russia fought a border war that ended in stalemate the preceding century. China also fared decently on land during the Sino-French war in the late 1800s. During the mid-1800s, China fought and won the Taiping rebellion, which was the bloodiest civil war in human history. These actions demonstrated that the Qing Dynasty was no Maratha confederacy, and could not simply be defeated and broken apart in land engagements. Europeans achieved limited victory in coastal regions against China by using the mobility provided by naval supremacy, but a sustained campaign to control territory would have ended badly.

Even more concerning were other European powers. The economic free for all in China later termed the "open door" was created due to the paranoia European powers had towards each other. China was to be an open market for all economic interests out of fear that one power would monopolize it. This understanding was militarily enforced on two occasions. In the first, Russia attempted to establish a protectionist trade zone in Manchuria, and Britain and the United States responded by financially and diplomatically backing Japan to expel Russia from China. In the second, Japan itself tried to do the same thing, and the United States responded by backing the Chiang Kai Shek regime and escalating tensions until the two powers were at war.

So to summarize, China was subjected to imperialism, but imperialism in the 19th century, while exploitative, was a lot "lighter" and more invisible than most imagine. Today, we tend to see a clear division between old colonial administrations that ended in the 1960s and 70s and the modern times, where there is no imperialism whatsoever. The reality is that the economic mechanisms of imperialism still exist today, and are rooted in the complex game of trade manipulation. Europeans won that game in China, and to push further would have been bloody and would have angered other European powers.

Sources:

Zurndorfer, Hariet. Imperialism, Globalization, and Public Finance: The Case of Late Qing China.

Eng, Robert. Economic Imperialism in China,

Brown, David. Partnership with China: Sino-Foreign Joint Ventures in Historical Perspective.

Ladds, Catherine. China and treaty‐port imperialism.