Final Fantasy VII Remake introduced Sephiroth far earlier than he appeared in Final Fantasy VII, and the developers revealed that it's due to the character's fame that he appeared so soon in the remake series.
In Final Fantasy VII, the existence of Sephiroth is only touched on throughout the Midgar section of the game. This changes when Cloud and his friends are freed from imprisonment within the Shinra Headquarters, as Sephiroth takes control of the Jenova body within the laboratory and murders President Shinra. The player doesn't get a look at Sephiroth until the Kalm flashback, which happens after the party leaves Midgar.
Final Fantasy VII Remake only covers the Midgar portion of Final Fantasy VII, which led many fans to believe that he would only have a brief post-credits sequence cameo in the game. This opinion changed when Sephiroth had significant appearances in the trailers for the game. Final Fantasy VII director/Final Fantasy VII Remake producer Yoshinori Kitase has revealed on the Square Enix website that Sephiroth's fame since the release of the original game prompted the development team to include him in the story from the start.
Sephiroth appears in FINAL FANTASY VII REMAKE much earlier than he does in the original game. Why did you make the decision to introduce him here?
(...) Sephiroth is this massively overarching presence that looms over the whole FINAL FANTASY VII saga. We wanted to make sure that aspect of him was present in this first game in the project - that’s why we have introduced him much earlier in the story now.
Sephiroth has become one of the most iconic characters in the Final Fantasy franchise and fans would be hard-pressed to find a villain from the series as beloved as he is. Sephiroth has appeared in numerous crossovers and has represented the darker side of Final Fantasy VII in the Dissidia series, due to his sheer popularity.
The developers of Final Fantasy VII could not have predicted that Sephiroth would have become one of the most popular villains in the series, which is why the Shinra Corporation starts out as such prominent antagonists during the Midgar section of the game. It might have been cool for the developers of Final Fantasy VII Remake to save Sephiroth for a Marvel movie post-credits scene, but the sheer love for the character means that Square Enix just couldn't resist milking the silver-haired swordsman for all he is worth.
Final Fantasy VII Remake is available now for PlayStation 4.
Source: Square Enix