Limits on numbers. Welcoming you as a tourist and welcoming you indefinitely are not the same thing. It's not a big population so wouldn't take a very large number of arrivals to impact public services like healthcare and schools.
In a way, they're pretty japanese.
Eventually your retirement system will crumble. Not tomorrow, not next year, but maybe a couple decades from now. And it's not something you can fix in a year or two.