Why the audio subcarrier is never transmitted with baseband analogue video signal?
Because the video is baseband. That means that it is NOT modulated onto a VHF or UHF carrier.  You are asking to take the audio modulated onto a HF carrier and combine it with the baseband video signal. And then to demodulate the audio back into baseband at the other end of the cable. That has the deadly combination of costing more, plus being lower quality.  How is that "better"?

Remember that most consumer "entertainment electronics" started out as audio which already had the Left and Right channel RCA connector interconnection scheme well established.  So it was easier to just baseband video on a separate RCA connector/cable.

Furthermore, audio and video signals very often travel by different paths and get treated differently by separate equipment both at the creation end of the chain as well as at the consumer en of the chain. So combining audio and video together is often more of a problem than a convenience. Here in the digital 21st century we must use ADDITIONAL gear to "break-out" audio from video, and then "re-combine" it, which frequently causes loss of A/V sync because it typically takes longer to process the much higher bandwidth video data than for the audio bitstream.