Because they know that the worse the crisis gets, the harsher the economical consequences for China will be.
Countries and citizens remember where the virus comes from, like they do for SARS and it they will put pressure on their governments to move production elsewhere.
I think China will be "punished" anyways, regardless when this crisis ends. There's no way globalization will continue in the future, governments will force companies to move production of essential supplies (food, medicine, raw materials) back locally to make their economies resilient to similar shocks in the future.
Fortunately, for China, the country is starting to have enough local consumption that it probably doesn't need to be just an export hub any more (but undoubtedly it will hurt).
Edit: this will, I predict, hurt Western companies & shareholders as well. I expect many companies to be much more rigorously stress-tested for supply chain and/or labour issues, similar to how banks are stress-tested for credit issues. This will mean thinner margins, lower profits.
Obviously, which is why I'm guessing it will be centrally enforced (laws by governments). Not for companies like Uber or Netflix, but whatever is consider "essential" (medicine, energy, food, raw metals, ...).
Frankly I'm on the side that the Chinese are acting in good faith here. They have every motivation to do so. Stamping out the virus worldwide is for them a first order national priority.
It’s not necessarily an act of charity. China has a vested interest in looking like the country capable of leading the world out of the pandemic, especially as Trump continues to stumble and alienate the rest of the world - it’s the same game of politics that all countries play all the time.
Crises are how rising powers become dominant powers - WW2 was the crisis that propelled America past UK to become the worlds dominant power.