Running the brake cables through the steerer tube and seat tube is not so easy as stem stack height and saddle height are different for individuals. So where does the manufacturer pre-drill the holes (assuming that it is as easy as drilling some holes to allow the cables to pass)?
Oh I meant the seatpost should have a slot, from the bottom upwards (not all the way, but far enough), in drive direction, letting the cable channel pass and still allowing normal seatpost adjustment.
As for the fork, there are nuts and bolts within the steerer tube that obstruct the clear passage of the brake cable. A fork that "internalizes" the front cable therefore has to have an external cowling/ cover outside of the headtube to create a channel to allow the cable to pass through.
I've seen forks with a clean tube inside, so it should be possible.
Anyway those mods are probably not very easy to put into mass production, but should be within the reach of skilled artisans. About the aerodynamic sigficance, not sure that would be noticable, let alone significant.