It's part of the "show" and part of the "control".
After the locals have spent millions on their new local Ideal Org, upper management needs to put on a show that they've done it. By staffing with Sea Org members and not newcomer-local-staffers, you can actually pretend you have an org by having it run by some people that know a little of the ropes. So then you recruit like hell and show the local-newcomers how wonderful and successful Scientology is; and you get them to join staff, try to train them and make them work. As the org gets manned-up with local-newcomers (and some oldtimers) you gradually re-assign your Sea Org members back to an established org, or move them on to the next soon-to-be-opened Ideal Org and continue the process.
The object is to get these orgs staffed with people who don't know any better - cheap labor, so to speak. There will always be some Sea Org members left behind to keep everyone in line and working. Every org that didn't have staff tightly controlled by David Miscavige et al gradually failed until it fell apart or closed.
You see, the admin tech doesn't work. It's the unwritten orders and push from uplines, coupled with the low wages, that makes any semblance of a money-making machine. Just having staff working towards a "theta world" and getting people trained and audited while using LRH Management Tech does not make a viable business.
You need the control mechanisms in place to keep the place productive.