The better flying schools do this anyway and having multiple exams ensures that such flexibility will be retained.
The flying schools can only have the three sets of exams that are issued by the CAA.
Even if they had more than three sets of exams this would not provide the kind of flexibility that is required.
Many potential PPL candidates have real lives with real businesses to run and real families to support.
For some the option of taking a short intensive ground school course followed by all of the exams, before starting the flying training is the option that best fits their "real life" needs.
Some schools include the PPL ground school as part of a degree course. This usually involves a structured program of classroom training within the first semester, with the flying being done at some later date. In many cases this is not just a case of "memorize then forget" but involves study that is of a far more detailed level than the minimum required by EASA.
Some people wish to get all of the ground school and exams done before going off to do an intensive flying course.
Your idea of staging the ground school and exams to match progress in the flying phase would prevent all of the above options. But it would tie candidates to doing the ground training and exams with the ATO at which they are doing their flying training. This would be a good money spinner for the larger ATOs but the smaller ones would be unable to provide such a service.
Your answer above just illustrates the fact that you you do not understand what is actually needed. The people that we are talking about are not 1950's RAF Cadets. They are paying customers.