I thought about that. It is possible that there is observer bias, and I think in the past the US would have had strong reasons to keep hacking efforts secret.

However, at this point, the US is starting to look very weak internationally, and a cyber-counterattack on China would show that the US government is not a sitting duck. Plus, China is even better at self-destructive witch hunts than the US is, so the release of news that China's government servers had been hacked would cause them a lot of internal strife. It would also lay bare a lot of their human rights abuses, which are far worse than the US's.