the USB connector is a box that is not open to the inside of the phone, at least every one i've seen. I've even seen a couple of phones where the box that held the USB connector also had a speaker in it. I guess a headphone jack could be put inside of a similar box, but the main difference is all the contacts in TRR plugs are spring metal that are attached to the outside of the plastic housing and poke through, and that's if the spec calls for any sort of reinforcement. The simplest headphone jacks are just spring metal in empty space.