They don't have an infinite supply of radar installations.

They are only just able to detect the aircraft at the end of its life.

The US can produce more f35s than china can produce radar installations. This aircraft is still in mass production, with many more being built and sold to many nations. Which likely can't be detected. If they could then that would be in the announcement.

It's a bigger deal to detect the newer and more widely available aircraft that can be launched from aircraft carriers. The F22 is an interceptor, primarily for defense. So it's less likely to be used where multiple hostile radar installations are in range.