It might be customs, or it might be "piracy" (which is bullshit, since you can get music easy from other places other than ripping from video games).

But it's probably because games like Rock Band and Guitar Hero actually offer full multitracks.

Each and every song in those games carries multiple audio files that make up one song. DLC that contain isolated guitar audio, isolated bass, multiple isolated stems for drums (usually overhead mics, snare, kick), isolated vocals, isolated keys, and more; Rock Band and Guitar Hero are a goddamn gold mine.

Some people use those separated stems for their remixes, and some people use them for practice (muting certain instruments). Some have even ripped the audio files and made them work with third party games such as Frets On Fire and Phase Shift. Like Guitar Hero, which was already completely ripped to PC, and all of the audio stems from the on-disc songs and DLC are available online.

Rock Band has all of its audio from the on-disc songs available online, but only some of the content on early DLC is available, ever since Harmonix changed the encryption after a bunch of the early Rock Band 1 DLC was uploaded online. However, people have found a way to get access to the contents of the post-encryption DLC; a tedious task that's done on a song-by-song basis.

Still, I can understand why the music industry is pretty weary in putting any of these games on PC. Especially if it's something as valuable as multitrack audio, which are usually kept away from the public and stored in a metaphorical "vault" (sometimes even a literal vault, especially for those bigger record companies).

Rocksmith doesn't have full multitracks, at least I think they don't (I know more about Rock Band/Guitar Hero than that game). They probably have separated audio stems for guitar/bass, then they mix-down everything else into one audio file, so I'd think they're pretty safe. The Rocksmith developers are probably more worried about custom songs, haha.