[crote]

The problem is that the different fabs are, well, different. While on a high level they are basically doing the same thing, the actual process itself can vary quite a bit.

While you start with a fairly generic design, the final production artifacts are highly specialized to the fab. The fab essentially provides a "library" of components which are used to translate the generic design into something ready for production.

Switching fabs means re-engineering your design, re-running prototypes, re-validating basically every single part of your chip. This is a process which can easily take many months, perhaps even a year. Not to mention significant engineering costs.

Doing all this once is acceptable because you can just keep using that single design for decades. That's why each individual chip can be so cheap: production cost is near-zero, and engineering cost can be amortized over a giant volume.