This is sort of surprising to me, for both Porsche and other makes. Loud bearings and roughness are associated with pitting failure, which is a fatigue mode called contact fatigue (as opposed to say, bending fatigue or torsional fatigue). Quite simply it’s a result of repeated cycles of pressure on the bearing race as it spins. The race will break in with a polished appearance and may later develop a matte frosted appearance. Eventually tiny pieces chunk away, leaving the race pitted and noisy. Run it long enough and it can even break into pieces (if you’re deaf).
Grit and repeated frequent impact can affect contact fatigue life. I’m not sure about preload or lubrication since they shouldn’t vary much. Otherwise it’s just the result of millions of revolutions, loading and unloading the race. The relationships between cyclic stress and life are a very well known part of a discipline called fracture mechanics. So it comes down to your car’s load history, bearing heat treatment, subsequent grinding (which if not we’ll controlled can “burn” or damage the case, reducing fatigue life), and most of all, design standards.
Porsche could size bearings to tolerate all these variables and still provide nearly infinite fatigue life. Pick your target: 100,000 miles? 200,000? It appears they’re aiming low but it does keep weight and initial cost down. To upsize the bearing to reduce contact stress, everything in the system gets bigger, heavier, and more expensive. Sounds like other auto makers have similar standards. Aircraft bearings are designed FAR more conservatively!