It's actually possible they don't have the source code anymore. Not sure what they used for FF VII ports; If they're using the PSX version converted to PS4/Steam or if they hacked the PC version to run on steam and made a fresh copy for PS4 from the PSX version. If they're using the PC version as the base for FFVII, then I'm sure they don't have the source code for FFVIII cause they're hacking the PC version to run on Steam. Not sure if the PSX code exists still or not.
Steam version is an adjusted version of the PC version so they still have the source code it's very likely just down to the copyright issue as VIII is the only FF to not even be remastered in anyway.