>But the question is, why do a country with such ability still want to spy United States? That ability should be more advanced than the current US technology for decades ...
Mainly because it's cheaper to let somebody else do the research and find the problems.
If you were interested in stealth aircraft technology, you might be glad to read about another country's research into Schiff base salts for paint. Or somebody else's modeling for multistatic radar aircraft detection.
It costs tons to do these sorts of things. If by spying, you could just eliminate half of the tech dev blind alleys, you've saved a lot of money and maybe time as well.
Spying is cheap: software and hardware cadets in a military academy learning to exploit specific systems. The main cost is wages, which are lower in China. For military tech, you need materials and engineering specialists. Often multimillion dollar prototypes just to find out where the problems with the techniques are. Specialized facilities for testing prototypes. De novo techniques for testing those facilities. Compare spywork, where it might be enough to bribe a single person in a chip fab to add a backdoor circuit.
Spying is not cheap, and the main cost is technology & science, not wages. Even in WWII the technology spies are definitely top tier geniuses with brilliant minds, not to mention now it's 2019 already.
Mainly because it's cheaper to let somebody else do the research and find the problems.
If you were interested in stealth aircraft technology, you might be glad to read about another country's research into Schiff base salts for paint. Or somebody else's modeling for multistatic radar aircraft detection.
It costs tons to do these sorts of things. If by spying, you could just eliminate half of the tech dev blind alleys, you've saved a lot of money and maybe time as well.