.aaaand we're back full circle to the argument that kids don't now how to sit in a classroom and study these days!
As I have said, OP, in your case I don't think this is a reasonable argument to apply to your privileged, rural, academic, privately-educated, neurotypical, non-SEN children with highly supportive parents who both support academic work and expose them to outdoor exercise.
So I agree with your initial premise that FOR YOUR CHILDREN it isn't worthwhile.
However, for a 5 year old with no previous school or pre-school education, who have had a life to date that is almost entirely free of discipline or structure, and have parents who are poorly educated or non-school supporting themselves, Forest School can be an important bridge, within the school day, to the alien (to them) world of sitting still, accepting adult instruction / guidance, developing fine motor skills and controlling gross motor skills, deferring gratification, taking turns, sharing.
So it is not a REPLACEMENT for sitting still and learning, but in some cases very useful PREPARATION for sitting still and learning.