The true reason is obviously Kishi is just too lazy to draw more kids. He only created the ones he did to be Boruto's age and be on teams that are his generation. There is no story reason for him to make more. How realistic is it for everyone that had kids to all have their kid within a year of each other. The answer is virtually none in story, but out of story it was done for plot convenience and nothing more. Naruto got a second kid not because he needed one because, I expect Himawari to be mostly irrelevant but because he is the main character unlike the others whose family has been more focused on and a family with two kids including a boy and a girl is more photogenic. That is all. Realistically Konoha is a society that just went through great death and destruction from the previous war and historically you would expect a baby boom after such but instead they can't even maintain their population. No wonder so many of the clans are dying out. No one in Kishi's story has any kids.