>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.