Why is the brake mount a separate part and not molded in to the carbon like most frames?

The machined caliper mount ensures that the caliper mounting faces are absolutely true to the axle bore centerline. Many carbon frame manufacturers struggle with getting perfect position and alignment of bonded/molded inserts. This guarantees better caliper location and easier adjustment. In addition, we've found that it's really nice to be able to remove the caliper from the frame with one (the forward) bolt and not lose alignment. This helps with tricky brake bleeds, or if you need to partially remove the rear triangle to access a bearing, fastener, etc.