Don't know what country you live in, but it's not America.

https://medium.com/fourth-wave/smashing-the-stereotype-women-always-win-in-family-court-c723a07cfe6

In her Huff Post article, Cathy Meyer does a stellar job breaking down exactly why this statistic is so misleading:

In 51 percent of custody cases, both parents agreed — on their own — that mom become the custodial parent.

In 29 percent of custody cases, the decision was made without any third party involvement.

In 11 percent of custody cases, the decision for mom to have custody was made during mediation.

In 5 percent of custody cases, the issue was resolved after a custody evaluation.

Only 4 percent of custody cases went to trial and of that 4 percent, only 1.5 percent completed custody litigation.In other words, 91 percent of child custody after divorce is decided with no interference from the family court system.

https://medium.com/fourth-wave/the-true-victims-of-the-family-court-ec6ee3309932

In a huge number of case, violent and abusive fathers get partial custody - and end up harming or killing their kids.