I mean, they're high school-aged. Even if they're at the latest of 18 years old, the latest research suggests a person's brain isn't fully developed until they're 25, on average. I think we can say that 18, while being the 'age of majority', is also the average age where people start to become more conscious of their actions, decisions, and the weight that comes with them.
The reason why most of these romcom mangas take place in high school years is it's because it's a time of whirlwind emotions, confusing situations, and most importantly, 'growing up'. Add into the complications of growing up in a single-parent household that neglects their own kid, let alone a married pair of parents letting a high schooler live on their own with no supervision, why wouldn't their emotional developments be stunted?