It's because those games have developed from a lineage of porn games that goes back a long time. It used to be that you could only find porn games that were either flash games, Japanese games, or TF (transformation) games. The TF game community has been making games about the same premise (you end up in some contrived situation where you are gradually turned from man into woman) since at least 2008. At least that's the earliest date publication date I found for a game on TFgames.site, which is the main gathering spot of that community.
Back in those days, 3D software was hideously expensive and difficult and took a real beast of a computer, and there were no game engines that were both decent and free. You couldn't even pirate a decent game engine. The best thing you could steal was Flash, which wasn't anywhere near decent. The free game engines were straight-up garbage fires and genetically closer to real-world Gonorrhoea than to anything playable. People actually preferred making games in the Warcraft III map editor (which is how DoTA was born). The only thing that was sorta available was RAGS, a way to play MUD (i.e. text) games but on a GUI. Nevermind the GUI looked like the control panel to an industrial pump station, it was something. And oh boy, did a particular demographic run with that something. You could even add images! As mentioned before, 3D was prohibitively difficult and expensive, and most people can't draw to save their lives, so the only convenient option was to just steal art or from other porn.
TF gamers, if you couldn't guess from the description earlier, are made up for a large part of trans people. At the time, the prospects for gender surgery were bad and the environment outright dismissive of transgenders, so it's not too surprising they took to making games about being literally transformed into a girl. Not all TF games have femdom, many even don't, but it's adjacent enough that it features regularly. Likewise, TF games have a broad spectrum of what is considered "transformation", going from simple cross-dressing to full-on drugs that trigger a chemical irriversible sex change that shrivels your dick right off. The sheer volume of games the TF community produced (relatively to non-TF games) back then meant that most western games were TF games, but if you were looking for either femdom or sissification, TF games were pretty much your only option.
In time, with the advent of Ren'Py and the porn game revolution that Akabur brought into motion, the share of TF games declined. But they've continued making games, and even in the face of better game engines like Ren'Py, RPG Maker, Unity, Godot, etc., many TF game creators have stuck to making text games to this day. At the same time many creators from the mainstream porn game scene have discovered TF games, especially those creators who were interested in femdom and sissification. So they've adopted much of the traditional TF game style, and this is why many text games have design elements and themes in common with TF games.
