Why hasnt a turntable manufacturer produced a table with automatic tonearm return/shutoff?

@kavakat1, several features have changed/evolved(!) over the years.

Tone controls were common into the '70s but became rare after that, except on receivers which were more of a consumer product rather than audio hobbyist.

Speakers once commonly had bass and/or treble switches to adjust other than flat.

And yes, into the '70s automatic turntables (start up and shut off) were common.

But about that time reviewers (led by J Gordon Holt, then Harry Pearson) became more critical of component performance and recommended those with fewer features which were considered detrimental to "pure sonics".  Manufacturers of higher quality components followed this criticism.

This is not to say all hobbyists wanted to eliminate these features.  Some continue to call for their return, you included.

Why?

Dual, Thorens have auto lif/off. Just more stuff to fix. 

Back in the day I had a Dual 1219 that stacked records and multi record sets were generally set up for stacking.  Record 1 had side a and d, and record 2 had side c and d, for example.  Operas were more complicated.  ;-)  I used the stacking feature at that time, still have all those records.  No harm came to them.  So the assumption that record stacking harms records is not axiomatic.  If I had a lot of old multi-record sets that were configured that way, and had a penchant for listening to them with any frequency, I would not hesitate to acquire and happily use a changer.  Both Dual and Miracord made good ones.  I know because I set up and cared for a lot of them for my happy customers.  With that said I can't imagine that tooling up to manufacture such a product today would pay dividends.

@billstevenson  I had that turntable, too! I think I used the multi spindle a few times but never took to it. Got a lot of mileage out of that old Dual.

You don't see it because the manufacturers don't believe there's enough demand for it to bother. Also, most audiophiles and manufacturers believe that it has a negative impact on sonic quality...and as others have mentioned, it's just something else to break.