There are some other reasons several major countries might not want to press the issue.
It would be an interesting international law mess. Some thorny issues:
1. If it was due to Chinese negligence, how would damages be calculated?
In many (most?) national law systems the party harmed by negligence has some duty to mitigate the harm. For instance if you are out watering your garden with a garden hose, and you see an ember from my BBQ land on your garden shed and start a fire which you could trivially put out with your hose, but you just stand there and watch the fire spread and burn the shed to the ground, I'm probably not going to be liable for buying you a new shed. My liability will probably be limited to the small amount of damage from the first few seconds of the fire.
Is there any similar concept in international law? If so, I expect that a lot of countries would want to avoid trying to answer the question of how much of their deaths and economic loses were due to China's negligence and how much were due to their own poor handling of the pandemic.
2. If it was an accidental lab release but not due to negligence, are there damages?
In many national legal systems there is the concept of an "Act of God" or similar. As an example suppose one of the 30 m (100 ft) tall trees in my neighbor's yard about 18 m (60 ft) from my house were to fall over in a windstorm and smash my house. If the tree was unhealthy or its root system shallow or weak, the neighbor would probably be negligent for not having the tree safely removed, and so my neighbor would owe me a new house. If the tree was a healthy tree well rooted then we'd be looking at an "Act of God", and my neighbor is off the hook.
Is there anything similar in international law?