If you substituted machined parts for castings you would have some very expensive locks that few would be able to afford. Even assuming you are using the best possible CNC mills with 4th and 5th axis options there is going to be a lot of handwork to say nothing of the time spent programming and creating jigs and fixtures (to say nothing about the expense of the machine and software to design with). Regardless of the wait, TRS, parts are still relatively cheap and their use of investment castings offers a comparatively inexpensive way to produce them while using processes that are well regarded in industry and in use by many leading firearms manufacturers.