From my experience in these clubs over 20 years, I disagree. Most kids don’t need their parent or sibling hovering around. Nor do they need them all day long in school. They do just fine and learn valuable skills of social interaction
I might agree with your “plenty” comment if we were talking about the clubs as they exist today. If they were separated according to small age ranges of 2-3 years with structured, age-appropriate activity most would be fine. Just like most are fine in school