Sek, it’s not shyness, but the behaviour stems from the manner in which they have been raised. Japanese culture is one in which, kids are brought up to respect parents, and elders. Maintaining peace amongst each other is highly valued. They are expected to conduct themselves in an honourable manner, they bow when greeting each other. Notice Naomi bows her head ever so slightly after a match, as she approaches the net. Nishikori does the same.
The traditional values are deeply engrained in most Japanese people raised in Japan. Even though Naomi lived most of her formative years in North America, her mother was raised to adulthood in Japan, and has passed the traditional values onto her daughters. They have also spent time in Japan with relatives there.
Apparently, Japanese men cheat as much as their North American and European counterparts do. However the divorce rate isn’t as high as in those countries afore mentioned. Wives want to keep the family together, and they value peace in the relationship, therefore they soldier on, even if the marriage devolves into a sexless relationship.
Important footnote*. Geisha girls aren’t hookers…lol. That’s a myth. I was the one who made that inference.