Part of it has to do with regulation, but a lot of it has to do with being part of the larger system. A lot of educators and administrators simply know no other way. They were brought up with the 8+ hour school day, each lesson being 45-60 minutes, quizzes every few weeks, workbook exercises, and so on. They merely parrot their own experience for their students, regardless of who is funding it (the government or the parents).

As well, you have to consider the end goal of primary & secondary education in most nations. The schools still direct students, for example, to take college entrance exams and whatnot, so everything is directed towards that final goal. If you're in the US, a lot of private schools still prepare students to do SATs or the like, just as their public school counterparts do. If you're in the UK or other Commonwealth nations, students are prepared for their GCSEs and A-Levels.

Can private schools offer this model to a higher level than government schools? Much of the time, certainly! Looking at class sizes, quality of teaching staff, facilities, and resources, most students are doing far better in private schools than public or government ones. Does that mean the fiat model of education should be our ideal? Definitely not!