They're doing that because they are traditionally secretive about mistakes in their back yard, and because there is zero upside to them finding the actual source. The CCP doesn't believe in the intrinsic value of openness in science and government.

It doesn't matter whether they did it on purpose like an evil mustache twirling villain, or some group of bats conspired to sully the good name of china, the rest of the world will still scream angry things at china. There's no way for them to win, so why bother.

Meanwhile the accusation of china doing bad things conveniently lets the rest of the world ignore the massive and blatant mistakes they made once the virus made it to their borders, as if an unfeeling chunk of proteins cares about national interests or what you call yourself