Not only was Ruby young when she died, she had spent most of her life in Hospitals.
So for her it's basically starting at 0 again, unlike her Brother who was a Doc.
The average person, struggling for survival on his small plot of land, will likely never have thought about the shape of the world after all.
There's no reason to, when the furthest you will go from your home is the next village or maybe - when your far-travelled - a bigger town in your region.
@Oshekebdkdbd
In Japan they actually have to a degree, there's a curfew for minors and while they are apparently not strictly enforced in most cities, I can totally see a teacher being a dick about it.
@heisenbergth
Easy answer, the author wanted the plot twist of the mothers turning on the villain.
Can't have that as a surprise, when the reader knows they planned it.
@Doomroar
There's limited space in the approach to Mirai I'd say, so they can't all attack at the same time, but you're right the plan sounded more bombastic.
Gotta love comment sections on RomComs, always acting like they don't know how this genre works and that we will have to dance this dance for however long the Author can drag it. xD