So why aren't other governments doing something similar?

Our government (Australia) basically told the last couple of brands that manufactured locally to piss off. So they did. So now we have literally zero local manufacturers.

We can literally do a second round of Cash for Clunkers and create American EV demand

US government does plenty of subsidies, not sure why you think China shouldn't do the same. The more power CPC wields internationally the less power burger empire wields. Seems like a win-win to me.

I think we need more govt subsidies to green tech and less subsidies to the fossil industry, i don't care if it's all a big 8d chess move to make us all speak Mandarin Chinese, the important bit is not dying rn.

China is definitely pumping money into its manufacturing capabilities, but I don't think cornering the international market is why.

What appears to be happening is that local officials pushed an industrial development policy to make economic numbers go up. Some provinces got to the point of building factories and leasing them out to companies for that sweet economic growth.

The Chinese national government is trying to find a market for this since it can't afford for all Chinese local investment to be bad. Developing countries don't have the funds for the cheap solar output and the other industrialized nations are putting up trade barriers since they are running massive trade deficits with China already.