Yes any HS kid can go out and run on their own at any time (going barefoot if need be) to train for XC, but there are normally additional constraints on that if you are a kid from an economically less privileged family. Those kids might have to watch their younger siblings, do household chores, fix their own meals, etc. because both of their parents are working. Those kids are more likely to have to take a bus to and from school which can easily take over an extra hour a day. And there are no special busses that will take them home after any sports practice so just participating in sports is harder.
The transportation thing is big. We hear all these stories of kids who grew up in Kenya and Ethiopia running miles each day to and from school. That is safer in a rural environment with less motorized traffic and space to be a pedestrian. Running for transportation is also more culturally and socially acceptable there.
In many areas of the US you would be risking your life if you tried to even walk to some schools. It would take a really strong and independent kid to survive the heckling they would receive for being so weird they ran to and from school.
Those East African kids ran to and from school as they had to minimize their time away from home as they had chores to do to help their parents and family.
Other kids with chores do not have the opportunity or instilled urgency to run to school.
In the US wealthier parents who are not both working and they drive their kids everywhere. When their kid is old enough they get them a car. Those parents are more likely to buy better food, provide and support opportunities for extracurricular activities, and even do things like hire a weekly house cleaner, baby sitters, etc which gives them more time and energy to support a running child.
It does not have to be but the road racing distance running in the US that a parent would do is essentially a white economically privileged sport. Those types of parents who run are much more likely to have the time, energy and initiative to encourage and support a running child.