There are specific reasons that this is the case, but most people will not discuss it openly. Putnam sat on his results for years because it is simply not acceptable to discuss in polite society.
Seems logical but social isolation is also increasing in Japan with their ethnic homogeneity. I think its all related to the collapse of large families. People have few children and when the kids grow up they're likely to move away.
When was the last time you were there? I spent most of the past month in metro Osaka, as a parent of young children, and I noted:
* Young children freely coming to city parks, fraternizing, and leaving, in absence of any adult supervision, indeed, without any adults other than myself even present
* Groups of young schoolchildren traveling mostly autonomously to and from school among busy urban streets (though sometimes under the watchful eye of many volunteer shepherds along the route)
* Children as young as 8 approaching me to be generally curious, maturely and completely without fear
* Proprietor of a local bar I visited knew everyone around, including my in-laws' family, this in the busy inner-city
* Ample resources and facilities provided by the prefectural and local governments, including staffed play areas and civic center events. My older daughter was broadcast on-air TV with other children attending a Girls' Day event.
* health care including vision/dental costs hard capped to 500 yen/treatment day (~$5) for children under 6
So yeah. I don't think the situation in Japan is comparable at all in any way to what's happened here in the USA. I would not allow even the appearance of my daughter playing alone in my own front yard, for fear of what well-intentioned neighbors might say or do, let alone the boogie-man.