Why else would China hide Covid related data from the WHO? Why don't they want the WHO to be able to pinpoint Covid's origins?
These are not intended to be comprehensive, and some overlap:
1) The Wuhan Institute of Virology is a major research centre. There may be stuff going on there China does not want anyone to know about, because it is commercially or - worse - national security sensitive. Bioweapon research is not impossible, or other things that might not look good, such as being unethical in other countries. Then there are new technologies, market products, etc. it wants to be ahead on and exploit. Thus there may be lots of reasons China does not want to give foreign investigators access to WIV that have nothing to do with covid-19.
2) China stands all sorts of risks of looking bad, even if covid-19 was not a lab leak. Investigators could well compile report that has the potential to make China or the CCP look deficient in some way - even at the bottom end: infection control, hygeine, healthcare, you name it.
3) China fundamentally, in every respect imaginable, rejects external scrutiny. China's conduct here is absolutely consistent with how it treats external scrutiny in any other respect- utter opposition. It rebukes countries for their way their free presses choose to report on China. It pretty bans people from entering just for talking to the Dalai Lama.
4) No country likes external scrutiny and avoids it wherever possible. It is arguably a form of loss of sovereignty, which countries guard jealously. The only reason there's a UN inspector been within hundreds of miles of any Iranian power plants is that a lot of powerful countries pressured it into a deal. The only countries that accept external scrutiny are ones pushed into it, or at so much risk of embarrassment if they don't that it's a lesser evil. China is too big to push, and has too little to gain given it's in the doghouse globally for letting covid-19 loose whether it was natural or not.
You just imagine a global pandemic that first arose in Tennessee. Do you think the USA would give the WHO free access, or do you think they'd tell the WHO to fuck right off, they've got their own internal investigation capabilities, thanks. My guess is the latter is more than a little plausible, especially depending on who's running the USA at the time. I can quite bet you, the entire right wing half of the USA would be frothing with fury at the sheer impudence of these grubby foreign interlopers from the WHO to even suggest that they should be allowed to judge the glorious US of A, that it was a plan to fit-up, embarrass and weaken the USA in front of the world.
5) The CCP fundamentally likes control. It wants to control what it's country does, what it's people think about the country, and what the rest of the world thinks about China. It mortally fears and resents criticism, especially internally from the Chinese themselves, and values stability. Remember that when covid-19 arose, it criminally charged its own citizens for talking about the new disease publicly. At the point it gives free rein to the WHO, it surrenders control. It is not what the CCP does.
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So, to answer any question like "Why would China refuse a WHO investigation if it had nothing to hide", the answer is simply that China's longstanding record of attitude and actions strongly suggest it would refuse a unconstrained external investigation on basic principle, irrespective of what the findings would be.