If (the villainess) didn't exist, it'd just be a game where (the game heroine) flirts and makes out with attractive men
The basis of most otome games are about vicariously romancing attractive guys via the game's heroine. Villainess love rivals don't usually exist in most games because they'd get in the way of the romance. Most drama tends to be generated from whatever tortured pasts/emotional issues the love interests have in their respective routes; the heroine then has to solve and heal them.
However, plot villains/villainesses are more common; and tend to be the underlying connecting thread accross most romance routes. Usually each route might contain pieces of the overall story, only to be fully revealed in the primary love interest's route. (Ex. in Collar x Malice, the full scope of the overall terrorist plot is laid bare in Yanagi's route, while others give you bits and pieces.)