And I would also guess that the main reason they don't do it is becuase it is a PIA to get to - it can take a good 30min + just to replace the air filter! I'm sure the average BMW garage can service 3 cars in that time
I did mine a while back and once you finally get to it, it looks like you have half the engine bay on the drive!
As Powers said, why change something the customer has no way of checking.