That's because the DRM is stripped, the DRM often have many compatibility/integrity/security issues etc. that pirates never have to worry about. They are not forced to watch through FBI warnings before watching a movie.
The experience of downloading something is vastly superior to buying it in most cases, and that's assuming you can buy it at all (pirates don't have to wait/hope for a TV series to be released outside of US, for instance).
Furthermore, you know things like Bitrate before you download. On Netflix, you don't know how a movie has been compressed. Even 1080p movies can have noticeable quality differences depending on the method of compression. Using a private tracker, you can see the exact program and settings used before downloading.
Don't buy into the RIAA/MPAA lies. Piracy is not just about getting stuff for free.