What's interesting is that the United States has prohibited goods produced by forced labour since 1930, so they're way ahead of us. That said, they've only systematically applied that ban for a few years.
Of course, it's understandable that there's also the political aspect. The issue of the Uighurs is very important. As you said, the United States has a law against the forced labour of the Uighurs. That's one of the reasons why things are much stricter at the border.
There's experience, but also the burden of proof. In the United States, if there's suspicion about certain goods, the importer can be required to prove that they weren't produced by forced labour before their distribution is authorized. There are laws and thresholds that are very different from those in effect in Canada, where privacy laws make it impossible to even say what ship was inspected and who the importer was. The evidence threshold is much higher, and it's the government that must investigate.