Many of the fakes almost work. Some are ghost parts. Some are recycled. Some are parts that failed QC or were binned as a lower tolerance part and remarked as a higher tolerance part. Testing for all those possibilities is not economically feasible, especially if you’re outsourcing assembly.

This is much more a “how do I know the 683rd chip on this reel will meet its sleep current specifications [so our device will make its battery life specs]?” rather than “how do I know this is an IC at all and not a fancy Lego brick?”