Part of it I imagine is because Westwood made the game and then got bought up and shutdown under EA. Asset tracking would be a mess.

Other part of it is most studios didn't imagine a use for old games in the future. So they weren't archived properly. World of Warcraft original source code was mostly lost and that game sold incredibly well and the company stayed in business. More modern studios are thinking more about remasters, remakes and archiving their work now so it's mostly a problem with older titles.