Why would I want a 3.5mm stereo audio socket ground designed so they connected to the casing?
On the contrary most of the 269 unique stereo 3.5mm jacks on Digikey's stock-on-hand are not what you assumed.


But , let me re-phrase your question.
Why would you want chassis ground to be audio common?
Often the chassis serves to shield the electronics with a low impedance low RF current signal node that qualifies as a low EMI signal ground.
The speaker common ground ought to have a low-impedance for low mV drop with shared current ground return to eliminate common-mode impedance rise and stereo crosstalk.
Also the use of mobile devices with a noisy USB Charger that is floating but with high impedance common mode noise from leakage capacitance in the 5V SMPS magnetics can also interfere with high impedance microphone inputs.
Also the hand capacitively couple a large surface body dielectric serves to absorb stray e-fields radiated by poorly shielded SMPS magnetics and suppress analog signal noise shared by the mike and earbud COMMON ground.
The goal is to raise the impedance to RF and SMPS audio modulated noise and make the inputs linear to this common mode noise so it does not get rectified as mike audio noise or lower the ground impedance and raise coupling to bypass this E-field antenna or H-field loop area inductive noise ingress.
They still do have what you describe in both plastic and metal, thread-mounted panel jacks.  which you failed to define in your question, but these are becoming less popular, yet give far better mechanical strain relief to the solder joints.
  Yet these cost over $3 vs the previous one much less $0.12 to $1.5.