The simple answer is: because it's working. Why would they abandon a policy that has been and continues to be incredibly successful? That's not to say there haven't been issues that have come up along the way, such as the massive corruption problem in the 90s and early 2000s, or the real estate bubble, or the out-of-control private tutoring industry.
Whenever such an issue appears which starts to seriously threaten social stability and negatively affect the positive trajectory that China is on, it is addressed and dealt with, as the aforementioned issues were. Other more minor issues are handled in a less top-down way and left to local governments to experiment and find the best solutions. China's approach is less ideological than maybe we would like and more practical, result-focused.
In addition to the general trend beginning in the late 1980s of decentralizing and delegating responsibilities to local governments, higher education in particular is a field where China has experienced a real revolution over the past 30-40 years, with an explosive growth in the number of students each year, and that can be hard for a government to deal with in a country as big as China while still maintaining high academic standards that let them compete internationally. For comparison:
China produces more STEM graduates each year than the entire Western world combined, and currently graduates about 12 million people each year in total, and yet its per capita GDP, even adjusted for PPP, is still lower than that of most European countries. So there is a huge amount of competition for a still not that high number of higher education spots considering the immense population size.
The way they currently deal with this challenge is by providing a lot of grants and scholarship programs for citizens from lower socio-economic or ethnic minority backgrounds, while letting those who can afford it pay their own tuition. Also, compared with other tuition systems it is still relatively cheap, because universities also receive extensive public subsidies, and because the vast majority of the system is essentially state run.
Here's a 2018 research paper on how financing of higher education in China has changed over the years: https://www.researchgate.net/publication/325171750_Higher_Education_Financing_in_China
As with everything in China if the current system starts to no longer be fit for purpose they will adapt and change. I can definitely foresee them going toward a tuition free model like some of the European countries if the current model begins to cause social issues, or impedes their technological and scientific advancement. I am definitely all for it, but China tends to be very conservative when it comes to making major changes when there is not a pressing need for them.