Perhaps the answer is just to intercept disk reads and stream on demand; 52x CD (fastest common speed) was only 6.3mb/sec and computers only had a few MB of RAM. Spread out over many hours of gameplay that’s not much at all. Later games that expected to be copied on entirety to the HDDn are a different matter, although I remember few of those. Honestly most games were a few tens of MB of game data and the rest were CDDA tracks. This was before mp3 and lossy audio compression became commonplace.

Circa late 90s a common feature was full or partial installs. A full install would copy everything to disk, but partial installs commonly kept the video and music files still on the CD.