In the days before engines and machinery, horses were required for transportation, farming, many other businesses, and war. Those are the things men were involved in. They used horses to accomplish a purpose rather than riding or raising them because they loved the animals. I'm speaking in generalities, I'm sure there were men at that time who loved horses as well as using them as a tool for work.
Today, at least in the United States, females are involved with horses in much greater numbers than males. Studies suggest 90% of those participating in horse activities are female. But, the top professional and international riders are often men—as are the top trainers and jockeys. Only 10% of jockeys are female.
At younger ages, I think riding horses in many areas has long been considered a girl-thing. That branding discourages some boys from riding. Boys are also enticed away by organized sports: football, basketball, and soccer as well as machines such as tractors, motorcycles, and cars.
Whatever the reason, there does seem to be an almost magical connection between girls and horses. I created this video years ago with photos and video of my own daughters and other girls in our 4-H club.