While the tax code may use a different definition for clarity, having a job is still technically running your own business in every other way. What is interesting is that while there are an infinite number of ways to make money, the vast majority of the population all choose to run their business in the exact same way (single client, set working hours, defined pay schedule, etc.). Is that business model chosen because it is the most efficient way to run a business, or is it because they learned to run their business that way from an external source, such as school?
I'm not sure we can garner any meaningful data from dropouts as only 8% of the population (in the US) drop out of the public education system, and I expect you'll find that the majority of them drop out in their later years, after already spending many years exposed to the system.
Re "set working hours": I think a lot of it goes back to early Industrial Age, when blue-collar people worked a lot more than now, as much as 14-16 hours/day. Most of their life was work, there wasn't any structure around it, and they worked for a single factory (the same factory, it didn't make sense to move around, or maybe there was only one factory in their town). Set working hours (limited at 8 hours/day) were a limitation imposed by the workers (or unions) against being over-worked (8hrs/day is much better than 14-16hrs/day). In most countries, this is still established by legislation.
Re "defined pay schedule": In some fields, you can get paid by a percentage-based commission (real estate, car sales, middle men in general, actors, movie directors etc). That's not always practical, and some employees prefer a fixed monthly salary (versus the insecurity of a percentage).
I'm not sure we can garner any meaningful data from dropouts as only 8% of the population (in the US) drop out of the public education system, and I expect you'll find that the majority of them drop out in their later years, after already spending many years exposed to the system.