Use the tanks we already have?
If actual combat kicks up, then tanks start getting destroyed or break down very quickly. If it takes a year to restart production, it's a very real possibility to not have enough before they start leaving the factory again.
Did you know that the Chinese state-run media is now hyping up nukes in preparation for a potential US war over COVID? [0] If they are actually working on increasing their nuke stockpile, they are definitely also working on increasing total military readiness.
If the US and China got into a conventional war, how long do you think the current tank stockpile would last? The US isn't going to take that chance and the very fact that they have a constant stream of available tanks is one more reason for China to avoid war.
On the whole, I'd rather pay a few dollars to help deter war rather than spend a bunch of lives fighting that war and in case the deterrent fails, at least we are ready.
[0] https://www.newsweek.com/china-state-media-says-country-must...
It's not just about welfare though.
If they shut the plants down and something happens, what do they do? The assembly lines will have been shut down or retrofitted for something else and the experienced workers may well have gone to work elsewhere.
They already have to keep the plant available for readiness and the workers need experience building actual tanks, so they keep pumping them out.