As the saying goes, "the poor man pays twice". The people does not just pay taxes. In a political sense, they also pay with their freedom, pay with their happiness, pay with their attention to pointless fuss, pay with their self esteem, pay with their health, and sometimes even pay with their lives. This answers the "why" from the payer's side.
On the receiver's side, the published/alleged amount of payment (whether it is true or not) is also a symbol for an attitude towards life and social relationships. According to confucianism, a leader mainly rules by what we would, today, call virtue signalling:
The Master said, "He who exercises government by means of his virtue
may be compared to the north polar star, which keeps its place and all
the stars turn towards it."
The chinese people ought to think, that Xi Jinping and his playmates are admirable for doing such a dutiful and responsible job for such a pitifully low salary. This clearly proves (according to that invincible logic), that Xi Jinping is a superior human being, that defers his own personal profit behind the wellbeing of his beloved people, and that he, his decisions and his lifelong leadership can be trusted beyond any imaginable doubt.
Seen from the perspective of money being just a means of effectively communicating power, Xi Jinping's nominally low income also illustrates the fact, that he can get things done without paying much. If he commanded a peasant to supply to him two trucks of starfruit to feed to his private pack of Powderpuff dogs, the peasant, who feels honored by this noble request, does not expect payment, because if he did, he would violate explicitly written codes of obedience, e.g. (from https://www.britannica.com/topic/xiao-Confucianism):
Xiao consists in putting the needs of parents and family elders over
self, spouse, and children, deferring to parents’ judgment, and
observing toward them the prescribed behavioral proprieties
Xi Jinping and the Communist Party of China serve as a substitute father and acting against them is strictly forbidden.
One might also speculate, based on anecdotal evidence, that a misguided peasant could well find himself in a reeducation camp, without the public ever hearing of him again, or his wife getting sterilized like we know from the Uyghurs, or getting killed for enforced organ transplants like we know from the Falun Gong practicitioners, if he dare to question the authority of the officials. Of course, the peasant is only a parable that shall illustrate the relationship between the chinese people and their turbo-patriarchical state.
Hence, it is quite natural to assume that the incentives of chinese apparatchiks are not so much to seek in the money they earn, but rather in the question, how authoritarian they can act without getting punished by yet another superior who deems himself more virtuous than the former. Consequently, publicly shaming other people for not being virtuous enough is a very widespread problem in contemporary chinese society (see e.g. the Human Flesh Search Engine).