Seems we have talked about this before heh....
Indeed the "why" almost singularly originates from their patrician class being unable to directly deal any defeat+revenge to Japan and IJA/IJN.
The whole resolution of that came only by the foreigner's action on Japan.
Specifically how they (chinese patricians and some bourgeoisie) were then co-opted and absorbed and re-deployed (and what this indulged, obscured and even silenced in various measures) by the CCP winning out against the ROC on the mainland.
Mao even went so far as to say that without Japan, he would have unlikely been unable to come to power....in a way almost justifying the atrocities inflicted on his people (this was all suppressed during the time to not kick up a fuss on the issue).
This is why Mao's PRC followed a certain distasteful cowardly foreign policy (given many victims were still alive to seek justice) with Japan during the cold war on this issue.
This is the way of the totalitarian, one that would take far more lives than the Japanese ever could or did.
Large parts of the CCP psyche downstream today all stems from the protracted sobering conclusion to the "century of humiliation" without any redemptive final victory (unlike say what the Soviets managed on Berlin) given the gravity of what was inflicted on China.
This is the lingering unique psyche hallmark among PRC leadership class (consciously or subconsciously).
This is precisely why USSR could de-stalinise for example (since final resolution was achieved with massive sacrifice on the aggressor with own hands and own trial and tribulation)...but this is complete anathema to PRC for Mao...as simply there was no transfer of anything relevant to the larger national aegis (like say the WW2 struggle w.r.t USSR) past the leadership cult.
@Viva_vietnamm can tell you about the similar lies promulgated by CCP/PLA with regards to their debacle in 1979 (on Vietnam's border) and the next 10 or so years of border conflict (that often got quite serious).
If we had a legacy but relevantly involved Soviet fella, they could also give similar account of the same set of lies before during and after Ussuri as well.
Overall PRC now is actually far more guarded w.r.t India (at this juncture) as it has lot more to lose now...the increase in ladakh flashpoint intensity was severe miscalculation by them and they are going through a certain style of "going through the motions" to both not escalate it but save face (especially for domestic audience past the journalists + people they squelched on weibo etc already) at same time.