Marriage and children (if chosen) are based on a belief in a better future.
The Japanese young people don’t see it.
What should give everyone pause is that these are intelligent and perceptive young people.
Japan is so very overly crowded with too many people and so very little elbow room that it just does not make sense to keep populating. After the population declines by 50% the young people will be more comfortable with starting families. There will be more room for the children to play.