How can it be that some old recordings sound sublime?

Yes real sound engineers but also I suppose real producers who let those engineers work. And good engineer knows how to choose equipment and use it. 

Class A tube mastering amps, tube microphones, minimal miking and a real sound engineer with real ears doing the work. Simple
enough as opposed to most of the dreck of today

THIS.....‼️

Celtic nailed it--no overdubs, heavy processing, etc.. But also minimal or judicious use of compression.  

A recording being mastered for home listening will not have the dynamic range compression of a recording that is anticipated for radio playback.  Heavy dynamic compression is used for radio playback because radio is often listened to in noisier environments--hence the need for "steady" volume levels.

Celtic66,
I believe that your  assessment is on the mark,  simple high quality equipment used in a simple /minimalist signal pathway. Sound engineers who seem to strive for a natural reproduction and relied on their ears. The vast majority of my jazz recordings from that era (1950s-1960s) are very good sounding. 

I don't listen to  much pop, rock or Hip hop, so can't comment on them . I can say unequivocally that most modern jazz recordings are done exceptionally well. As a jazz devotee I'm very happy with this outcome. It seems that the tendency to "over manipulate " or process the sound is avoided for this genre. I will say  that modern jazz recordings seem to get a fuller scale or weight from pianos than earlier recordings. The earlier era stereo recordings do have an undeniable natural ease and flow of the music.  I think that in most audio applications, simplicity has a high correlation with good natural sound quality. 
Charles, 

Modern live recordings aside, you're talking about two very different things.  Once you get very far beyond two mikes and a live recording you're putting together pieces of a puzzle rather than taking a snapshot of an actual event.  Non-live with multiple overdubs makes for a tricky process that is not likely to result in a natural sound.  There's also a difference between recording/reproducing acoustic versus electric sounds.  I, too, am often surprised if not astonished at the great sound of some recordings from the fifties, mono and stereo alike.