The main issue is that as a government you also don't necessarily want to provide your population with everything certain parts of the population want to consume because it will create negative externalities. That's why stuff like drunk driving, drug possession, or gambling are banned and criminalized. Mass suburbanization causing power usage to increase due to detached housing being less energy efficient than apartment complexes is an example of this. Difficulty with managing scarce land resources when China's population density is 4x that of the suburbanized US (8x if you look only east of the Heihe-Tengchong line) is another issue.
And the thing is China's urban real estate market is heavily commodified, prices generally indicate where people want to live. A large part of this is influenced by the family registration system and parents wanting to change residency to places with better education resources and the inequitable distribution of these resources, which is a legitimate issue in China that the government is trying to fix with their unified national market strategy. This results in the highest home prices being in the core of tier 1 cities. Chinese families definitely prioritize other factors than living space/yard space primarily when choosing a home (not saying it's not a factor but not the primary one).
The lack of sub-urbanization in China is a direct product of civil planning. The hukou system, the concentration of education, jobs, and infrastructure, the prioritization of coastal (e.g. exports-driven) development, among other factors. Also, China deliberately built up its economic hubs, there was nothing unintentional about it.
But China isn't Japan or South Korea. It doesn't actually have a lack of space. If you want to use the US as a comparison, most of the US is under-populated. If you ever went to the Mid-West or the South, you'd understand how much of the space in the US is not or barely developed. China may have more population, but it is extremely concentrated into certain regions, while in most of the country, the population density is actually lower than Europe. It definitely has the space to sub-urbanize.
Failure to do so will have consequences. Continued emigration is one of them - the US may have done China a favor by becoming unfriendly to immigration, but there's plenty of other countries elite Chinese are heading to - Canada, Australia, New Zealand, among others. Another consequence is demographics - same situation as in Japan and South Korea, people don't want to have children in super dense cities where they barely have enough living space themselves. All of this might be solvable via policy, but I brought this up primarily because you seemed to imply that living quality was somehow inversely correlated with large living spaces, when every Chinese person I've ever spoken to indicated the opposite opinion.