Why high-end cable manufacturers don't post measurements?

It is only partially true. Most of the manufacturers I have seen do post measurements. Of the length. Some, rare ones I think, do even post gauge. That would be about it, but is hard to catch them saying "never post measurements".

Because there aren't any measurable differences between any reasonably designed cables.

Check out http://www.ielogical.com/Audio/CableSnakeOil.php for how cables interact with components. Be sure to read the Garen Galeis papers. Also https://www.stereophile.com/content/what-difference-wire-makes from 1995.

Nor any sonic differences that can be readily identified blind.

It depends on the system. Some systems will confound the most skilled listeners. Some systems are relatively immune to cable effects. Other combinations are like flipping a switch.

Thank you ieales.  Great info.

"So are some of the prices—with exotic speaker cables ranging up to $100/m, or even more!"

What a difference 25 years make.

https://www.stereophile.com/content/what-difference-wire-makes

Let me know when one can measure tonality and soundstage of a cable with an oscilloscope. I’ll wait.