She's not, but it's the most prominent way to do it. And Cloud is the main character, plus she already has interest in him. So it's just extrapolating from what's already there, not making things up out of whole cloth, and the romance options in FF7 have always been from his view, that is, to be with him, not with the other characters. Aerith and Tifa were his choices, not someone else's. So it wouldn't make sense to put Biggs and Wedge on that list. I don't know why people bring that up.
And the trailers have given subtle hints of romantic potential between Cloud and Jess, especially the TGS trailer. When they first get on the bikes, before it cuts to the tunnel and Blondie's entrance, we get a head-on closeup view of Cloud and Jess on their bike for a full several seconds before the scene shifts. They could have shot this scene from any number of angles or kept it featuring all four of them, but they didn't. SE deliberately chose a shot for that part that highlighted them both, and only them, and we don't know how long that shot actually goes on for in the game itself. During the parachuting, Jess is clearly happy that Cloud's sticking around, and he seems to hear her, judging from the cut to him that we see while she and Biggs are talking about him. Combine those things with the scene of her giving him the materia in the E3 trailer plus her increased flirtatiousness and the fact of her existing feelings for him and the possibility of her being a love interest just as viable as Aerith and Tifa for Part 1 isn't so farfetched.
RN, romance is a part of stories, and it can be done well and be effective and used to explore other parts of a character's personality and story. Otherwise it would never be used. Should the OG have never had Aerith and Tifa feeling the way they do about him at all? And Jessie's not defenseless. Just watch her during the pillar segment, she's fighting alongside everyone else. Plus you forget she takes out one of the train guards at the start of the game with a swift kick to the chest. She can take care of herself.
Rog, I already mentioned a couple possible ways that could be handled. She could have dodged the trapdoor and escaped on her own or been sent on ahead through the sewers by Cloud while he and other two fought Aps. And there are probably other possibilities as well that could still be feasible. So just because something doesn't spring instantly to mind doesn't mean it isn't possible. I did say her appearance there was unlikely, I was just trying to make the point that theoretically, it could still be made to work if she was.