It's the same as any product, the product team wants a faster, cheaper product, yesterday. Security and trust is secondary, because if you're lucky enough, that will fall on the next product team.

Beyond that, processors contain billions (or trillions?) of possible outcomes from a set of inputs. Testing for all of these just to verify reliability and stamp out logic bugs is really hard due to the combinatorial explosion. Putting security testing on top just complicates matter further. The best they can probably do is map out potential ways in which their general purpose processors could be used for specific nefarious uses.