The amount of time children spend playing outside has
drastically decreased over the last generation:

Japanese people in their thirties, forties, and fifties used to play outdoors, on average, for more than hour and a half when they were kids.
Due to the growth of video games, and change in kids' lifestyle - such as increased participation in after-school activities, today's children in Japan spend, on average, less than an hour playing outside.
Simultaniously, the physical abilities of Japanese
children detoriated tremendously:
Yamanashi University conducted research comparing physical development of children of different generations by looking at performance in a variety of athletic disciplines including running, jumping, etc.
Shockingly, it was found that the average 5 year old child in 2007 had the athletic abilities of a 3 year old kid in 1985.
Similarly, the average third and fourth grader in 2007 was at the level of a 5 year old child in 1985.

While there are plenty of parks in Japan, many of
them are not ideal environments for free play due to:

UNSTIMULATIVE ENVIRONMENT

EXCESSIVE RULES
On top of that, the government lacks funds to renovate
old and broken play equipment:

According to a survey conducted by the Ministry of Land, Infrastructure, Transport and Tourism, 47.7% of Japan's approx. 464,000 pieces of play equipment are 20 years or older - with significant portion of those being broken or otherwise unusable.
As reported by Nikkei in December 2017, the government lacks the funds necessary to repair or replace that equipment - and is relying on support of donors more and more.