The high reject rate for reviews tends to be down to them wanting a lens that is perfect for the test body they use. The tolerances for lenses and bodies can either add up to something unacceptable or cancel each other out. For instance if a lens is at one end of a tolerance limit and the camera body is also slightly out in the same direction this ends up with a poor result. However if the body is perfect or out in the other direction this will produce a good result. Same lens, just bad luck if you get a poor result. This why getting another copy of lens often provides a good result. Many higher end bodies now include micro adjustments for focus because this very often can correct for tolerance mismatches. Even the L lenses that I own apart from one all needed a bit of micro adjustment to get spot on focus.

This is not to say that all problems are down to this phenomenon, there are bad ones out there but high end lenses that are actually defective are quite rare and even many of these can be sorted out with calibration adjustments.

I suspect that almost all lenses leave the factory in a good state but after being thrown around by couriers can end up needing recalibration.