It is because men aren’t engaged in parenting nearly as much as anecdotes suggest. I used to work at a family court and like 19 out of 20 men never even bothered to appear in their custody cases. When you go on reddit it’s a huge vast sexist conspiracy against men but in real life they fail to do the absolute bare minimum.
There aren’t any books about it because you are the 1 out of 20 dude and a large percent of that cohort doesn’t buy books.