Let's try to remember we're talking about a leisure activity here, not a career that requires a PhD. The motor skills (and teaching) requirements of the DM course are not particularly difficult for people with an aquatics (or education) background and can easily be mastered before ever considering going pro.
Since many dive pro candidates (DM is a professional level) are ill-prepared for the challenge, longer DM internships are often necessary to create a useful product and LDS owners love the free labor. By dragging it out over 3-6 months, they get your labor for a long period of time.
Somebody who is completely prepared, prior to the course, has little need for an internship that lasts half a year or longer --and charges 25% higher tuition rates-- because they're local. To me, a LDS that needs that long to create learning opportunities is either disorganized or cannot provide enough work to justify doing the DM with them. They're also using their candidates as unpaid staff, which is ethically ambiguous for a course that requires tuition unless you plan to work for that specific dive center.
Assuming you are prepared, both academically and physically, a 'factory' can easily provide you with ample access to students and the professional dive world, and they work with professional dive training nearly every day. They're also usually located in places that offer amazing dive opportunities for you to broaden your horizons, which is important to overall growth.
There is hardly a job in the world that does not require their own training after hiring, so bypassing a few things like learning the retail store end of the business should not be considered a huge negative. In fact, some might say its actually a positive.
For example, a new lifeguard with no experience is often preferred by aquatics managers because they are less rigid and opinionated about their work, and a 'green' employee is usually easier to train and has fewer bad habits.