Why don't frame manufacturers ream/face their frames?
First thing that comes to my mind is labour costs.  

Second thing that comes to mind is that once you break that paint seal by reaming/facing you are allowing contact with the air and elements.  As a good practice you should coat the threads with a little grease before assembly, meaning that when you get that new frame, you tap out the mounting holes and then screw in the mounting bolt with grease that will re-seal the exposed metal. 

Third..tradition.  New frames never came faced in the old days as it was expected that the professional bike mechanic would be doing all that as it was their job.