Outside of the two online Final Fantasy games, only two main entries of the Final Fantasy franchise ever found release on the PC. Final Fantasy VII and Final Fantasy VIII were originally released in 1998 and 2000, ported by now-Square Enix subsidary Eidos. While they were technically flawed, the ports helped introduced the franchise to a whole new audience.
It appears that Square Enix attempted to revive those ports for re-release on Steam. A report by RPG Site shows that Steam's "Content Description Record" (accessible via the "Steam Registry Toolkit") holds entries for both releases. It's clear that Square Enix had the intent to distribute the games on Valve's digital distribution service, as modifications to the game code were apparently discovered. According to the report, game files were edited, likely in an attempt to make the games "run better on newer operating systems."
However, it appears those attempts were unsuccessful, with RPG Site adding that the performance of the two games is "extremely temperamental" on modern systems. The last time Square Enix edited those files was in 2010--and it's unlikely that this project will be revisited. For now, it seems like fans of the games will have to rely on the PlayStation Network for a downloadable version that runs on PS3 and PSP.