They do receive takedown notices, however files uploaded to usenet are mirrored across many providers across many jurisdictions while also split into many parts as you noted. Usenets implementation of file sharing is quite robust; being able to rebuild a file that’s missing a significant portion of it’s data. To successfully take down a file, you need to remove many of these parts across almost all of the usenet backbones which requires cooperation across many nations/jurisdictions that are governed by varying laws. It’s not an easy task.
Here’s a somewhat limited map of usenet providers: