What do you mean why?
This memory stock popping on IPO has nothing to do with how well/bad capitalism provides in the USA. This is entirely about Chinese investors speculating on this company's ability to grow and undercut some Samsung, SK Hynix, Micron. Nobody knows on IPO day if a company will grow into the expectations of the investors.
And China's population mostly invested in Evergrande real estate, not stocks. Lots of workers in the US are at least invested in the stock market via their 401k/IRA, but the same isn't true of Chinese workers.
Also worth reiterating that most US stocks aren't doing great. The top 10 by market cap keep dragging the indexes into growth, but success is heavily biased towards the monster caps.
In the US, shareholders get rich while regular people get left behind. In China, shareholders returns seem quite bad, and standards of living seem to actually improve. Granted, China started from a much lower point, which makes rapid growth a lot easier.