@ligerhero It's only becoming more and more common here in the West too. Something like 30% of people between 25-29 were still living at home in 2016. It's pretty simple too in most cases: too damn expensive to move out. Everyone I know from before I graduated grew up hearing how it was "weird," or "creepy," or whatever for adults to live with their parents for too long, but a lot of us just can't afford to move out as soon as people used to most of the time now.
The cost of living on your own in my state is not really manageable for most people fresh out of school. Those who are already established enough in their careers may be able to afford it. Some of the people I know had to move out of state to find more affordable living costs in less crowded areas, but that's not a perfect solution for everyone either.
Meanwhile in Singapore apparently the 2016 figures were around 97% for people aged 15-35 living with their family if they weren't marred yet. I'm not sure what the deal is in Japan on this issue culturally speaking, but this sort of thing exists on a spectrum as far as normalcy goes.