the leaks occur because Apple begins manufacturing a month or two before the announcement.
More like one month tops and from the launch not the announcement.
Months and months of production means stuff in warehouses to get damaged or stolen. Plus draining batteries etc. And it means having to have iOS ready way further ahead than now.
Apple has been very upfront that they don't work in that big stock piling game. Which is part of why folks are so antsy about hitting the site the moment preorders go up, camping on lines etc. They know supplies will be very limited.