I've never understood this logic. If the game is good then why do you care ?
Because most devs can't script for shit, make crappy systems/gameplay loops, or in general slow or buggy messes, and with Ren'py you can fix the game on the fly. Dev abuses CPS or pauses? replace them all in 30 seconds and save literal hours of your life. Dev likes to gate crap behind repeating a variable 50 times? shorten the loop to a length that makes sense or edit the variables manually without messing with core game files like Assembly-Csharp or whatever it's called in Unity games.
Dev fuck up a label jump, transition, variable increment, etc.? Fix all that stuff on your own! Also you can see what you're getting into more easily before hand. Sure, you can extract assets from unity games but it's much more of a pain in the ass so you know if you're about to spend 3 hours roaming around for a crappy handjob scene with a mediocre story. Plus Ren'py is much smoother for VNs, has easily customizable menu, rollback, skip, save, textbox options etc. I have a ton of UI changes I make to Ren'py games on the fly to make them bearable if the dev has no clue what to do or tries to make their game artificially prohibitive. We've all only got so many hours in the day, I'm not gonna light them on fire bug testing or suffering at the hands of some shitty UX made by some amateur dev who has no clue WTF they are doing. Burned enough time doing that when I first got into this space and didn't know how bad most of the people who try this at all are.
There are a handful of decent Unity games on here and most of them aren't VNs. There is almost no reason to make a VN in Unity and that alone is enough to arouse suspicion in anyone who's been in this space long enough.