Theory: The “blind spot” feature is very reliant on the cameras for detecting cars (No bumper radar like other manufacturers). Prior to the most recent Autopilot releases, the stitching together of images from multiple cameras was less than stellar based on the jumping around of vehicles on the display. This impacts the cars environmental awareness and Tesla’s confidence in the system. They didn’t want people to rely on their blind spot system due to the occasional misidentification of vehicles. Therefore, no beep warning yet (a really easy feature to implement). I have a feeling we‘ll get an audio warning at some point.