Because 100% of units get the dash radio, so 100% of the units get tied into the chassis antenna. Only a small percentage get an outside radio, so to try and have them tie into that antenna and add another connector and more wiring from the dash is asking for other troubles
PLEASE do not send questions via Private Message, my inbox fills up too quickly. Post questions in the forum as a new thread so that other users can benefit from the answers. Use Private Messages for sensitive information only (VIN, Contact info or things you do not want the public to see).