Some kind of DRM on the disc (not the game data, but "outside" of it) so that it reads as an "official copy"? Just a guess, as I've never tried myself.
Like, some kind of stamp on the disc that the device reads. Such a stamp wouldn't be contained in the data that you could extract or burn to another disc.
Just a guess.
Edit: after doing a few seconds of research, Wikipedia says:
A map file that contains all of the exact positions and file size info of the disc is stored at a position that is beyond the file limit. The game calls this place directly so that burned copy with no data beyond file limit cannot be played.
The authority pattern pressed on internal circumference of the media, which could not be copied, is used to detect authorized copies. Some titles also use the Libcrypt mechanism to validate the disc by using checksum as magic number to subroutines.
People did it all the time back in the day - at least, here in thirdworldia. I know I played a lot of burned disks on my Xbox.
As I recall (it's been longer than I care to think about), you had to mod it so it would work; I remember having to take it to some skeevy electronics repair shop to have an "unlocking" chip installed.