Pre-filter cuts down the oil input line pressure to clean effectively. Secondly because the turbo is considered “internal”, the engine filter SHOULD be enough to keep the oil clean. If there was a pre-filter that would just be an additional filter for the owner to overlook at every oil change.
And of course, a lot of manufacturers just want the engine to die so they can get more money to repair it or make another sale. Thats the major difference between cars now and cars 30+ years ago.