@goodhunter it's just another "convenient anti-realism" thing.
It has very little to do with real life, just a convenient setup to make things happen while ignoring both the obvious and common sense.
Although Japan is so far-removed from closeness that it's considered fantasy to have stuff like this happen, which probably adds to why it happens in manga so much.
It's the same with being instantly on a first name basis with a girl, having a (female) childhood friend that goes in your house to wake you up every day, having several girls be attracted to you despite being ordinary simply because you're "kind" and teenage preachiness having remotely any effect on adult characters. These are all things that would never ever happen in real life Japan for the most part. So it's used as entertainment in fiction.