Social interaction is not synonymous with the office. Adults can choose to socially interact with any group (family, friends, shared interests) that they want to.
In theory kids can do the same too, but in practice, you'd need some set of adults to get all those kids into the same place somehow, another set of adults to supervise them so they are safe etc... at which point it becomes more efficient to combine this with schooling.
Also with working adults, there is an explicit expectation of delivery. Both the employer and the employee knows of the consequences for performance or non-performance, and part of being an adult is that you are aware of those consequences and choose to behave accordingly. With kids, such expectations aren't realistic.
Adults either do not have to learn that anymore, or are hopeless anyways and won't learn it ever.