So many suppliers and manufacturers in the supply chain, each with a workforce across many different roles...
I am surprised how few leaks we get of products. It does seem that the majority of companies, and people, have integrity.
And, when you think about it, hardware leaks are the least important leaks. Yes, hardware leaks are what fanboys (such as myself) like to see. And they guarantee page hits for sites like MacRumors. But the hardware is just the tip of the iceberg. The box the software comes in.
The real meat is the infrastructure (server farms), ecosystem (iTunes, App Store), operating system (iOS), the apps themselves, all of which add up to that intangible yet crucial element: the user experience. And all of that is impossible to copy overnight. And I'm pretty sure stories about leaked infrastructure or next-gen APIs don't generate nearly the web traffic that photos of iPhone backshells do. (Real or fake.)
Sure, there is the occasional hardware surprise like the 64-bit A7 last year. But by the time MacRumors posts artist renderings and 3d-printed mockups of this year's iPhone, Apple has already been working on next year's iPhone for about a year. In other words, I wouldn't be surprised if Apple started work on the 2015 iPhone in 2013.