We won't ever have an answer to that question, but the same engineer or group of engineers who chose a sealed bearing also put them into the manual gearboxes used in the models equipped with the M96 engine.
Likely it boils down to what is considered an acceptable lifespan for the bearing and acceptable failure rate. With a powertrain warranty of 5 years or 60,000 miles, it was likely figured these cars would be out of warranty before any problem occurred and they were most concerned with low cost of ownership and most buyers trading in on a new Porsche every 3-5 years. Manufacturers rate seals, greases, and other specifications for their parts, like bearings, with an MTBF, or mean time between failure.
On paper, these bearings probably should have never failed, but we all know that in the real world, components can respond differently than in simulation.