- the kids may not be "just walking around"
- may not be reading a sticker book
- may not be yelling at each other
- may not be finger painting
Why is any of that good? Do you need to see a neat line of kids quietly sitting in desks before you can assume any learning or anything valuable is taking place? Even if the students were being forced to drill their times tables, you can be sure that not all kids are going to figure it out. But probably the kid who might have learned 7th grade math will not actually learn 7th grade math---if anything, they'll likely get bored because things are moving too slow for them and they're forced to spend time doing something that they perceive to be a waste of time. I don't think there's anything inherently better about this situation.
I think you are biased by your notion of what school should look like.
Here are two famous people that went to Montessori schools: Sergei Brin: https://www.youtube.com/watch?v=BHzj3LrXruY Jeff Bezos: https://www.youtube.com/watch?v=Ox49EB1tNW4