Because, the steep transition that these cadet schemes require goes well beyond simply licenses, hours and rules.
Airlines that operate ab-initio cadet schemes want candidates who are likely to become on-line and productive within a time/cost budget. It is expensive to train cadets who themselves are on an extremely steep learning curve. It is therefore very important that those candidates are likely to succeed without a great deal of additional or remedial training. It is consequently very important that there is a low attrition rate.
The principle FTO's in these types of cadet programme, not only screen the most likely candidates, but they also provide full time, monitored and verifiable training to the airlines. They do that in a format which is usually compatible with their partner airlines own training regimes. The training itself not only leads to licence issue, but it does so in an environment which is tailored to the same operating philosophies as many of the airline customers.
The airline wants a seamless, continuous, and full time trained candidate. They want someone who is not likely (in most cases) to find the transition to their own operations overwhelming or unduly difficult. The airline expects the candidate to be well prepared for this transition. It also expects the FTO to provide cadets to a consistently high standard, such that the attrition (or failure) rate is kept low.
At this level, having the same licence, passing the same exams, having the required hours simply isn't enough. These programmes represent a fast track route into the pinnacle levels of the profession, and it is important for those companies offering the programmes, that the selected candidates are not only those most likely to be successful, but also those that have trained in the methodology and environments that is likely to ensure that success from their point of view.