I think it’s probably has to do with the way Korea doesn’t really have more than 1 way to (casually) refer to your dad beside Appa. Like in English we can call one dad and the other papa or something but Korea kinda only has appa so it would be confusing to call them both the same(?)
Am I the only one who's uncomfortable about everyone calling hyesung mom , and not dad or papa...like he's a man no ? It's just my opinion tho