it's not just personal profit, I mean if it's true how would you react to it? Putting aside how it would affect research, it also would make many people including such in power blame China for it. Which would be a political nightmare not just for China but also US, EU, etc.
So if any secret service has proof it was a leak you can be sue they wait until there is a major conflict with China. Like e.g. because China attacking Taiwan before the US/EU become more independent of TSMC. Through at that point of time there is also a lot of insensitive to just make up a "proof". So the public probably will never know for sure.
https://en.wikipedia.org/wiki/List_of_laboratory_biosecurity...
SARS itself leaked from the lab 4 times, once from a BSL-4 lab.
There are very small chances of many catastrophes that we happily ignore.
Among the less small ones, off the top of my head: a large earthquake on the San Andreas fault and the Yellowstone supervolcano erupting.
Can we prevent a lab leak? Not really insofar as humans are less than perfect individuals.
Pretending we hadn't discovered atomic fission would have been just as dangerous as ignoring biological threats.
What kind of evidence do you need? This paper is full of evidence that if you combine the probability of each event happening, the probability that it was accidental is really small. So the burden of probability should be to prove that it wasn't leaked which isn't proven after 3 years.