@lolnope I wouldn't say there's never any villainess'. To my understanding its not precisely a uncommon trope in them. Though the annulment thing obviously only happens in a story where that makes sense to happen. Its also a very simple way to up the stakes in something like that where the person you like has someone else aiming at them, then the engagement trope here can either be applied as a situation of 'after they'd already fallen for each other one or the others parents decided on a marriage with someone else. Fight fate.' or 'It turns out they already have a Fiance and because if the other person was a good person the player would have problems obviously breaking between two people and breaking their marriage they need to at least look like a bad person in this route.'.
Then because these stories are commonly based on older games and tropes most of them are like 'its strange the villainess is so 2 dimensional. Lets come up with a good reason why she seems like a bad guy. Maybe she's just Tsundere and the player doesn't get through that first wall, maybe they have a bad past that if you fix it they'd never have gone so far as to be killed in the end at least, maybe they aren't bad at all and the heroine is actually the bad guy and you just can't see it from your side of the game.'(That last is honestly my favorite right now with the story where the villainess is now running her families territory and it turned out the heroine going for the harem ending was actually a spy from a opposing kingdom trying to gain power and cause the end of the kingdom but you never saw her origin or what happens past the wedding in the original game and get no real mentions of the war brewing on the borders of the kingdom she's demilitarizing.)