So who do you think carried out these attacks? Do you think that China does not carry out any offensive hacking? Do you think they do, but avoid the US for some reason?
IMHO, these allegations are plausible enough to believe without strong evidence to the contrary. Taking the experience with the Iraqi WMD allegations as your North Star (to the exclusion of all other factors) seems like a heuristic that will be wrong far more often than it's right, and more often wrong than alternative heuristics.
If they claim to have evidence but don't provide it, I assume they don't have evidence, or that the evidence is weaker than they are claiming. If they do provide evidence, I consider the possibility that it has been tampered with, that its provenance is dubious, or that contrary evidence has been concealed.
We're talking about professional liars here. Not everything they say is wrong, but everything they say is suspect.
After that experience, I'll believe the US government only when they make all their evidence public, and even then, I'll be exceedingly skeptical.