Why is "withhold," but not "threshold," spelled with "hh"?

After all, they’re both words of plain Anglo-Saxon origin and they are pronounced almost the same.

Well, I would pronounce the second “h” in “withhold” as an /h/, but I don’t pronounce “threshold” with an /h/ sound. That’s probably because it’s no longer seen as formed by “thresh” + “hold”.

Because English spelling isn’t logical. The fact that one keeps the “h” has no bearing on whether “threshold” does not. By consensus, that’s how the spelling evolved.

The OED reports one-h spellings of “withhold” from as late as the 15th century, but the “hold” is at least genuinely from the word “hold.” That’s not the case with “threshhold,” which has at least as many historic and dialectal forms in “-wold” as “-hold.” OED says the second part is “doubtful,” though given the cognates obviously Germanic.

But as stated above, even if they were the same, that’s no guarantee of a consistent spelling.

No? I do, and I’ve always heard it that way.

Me too, but that appears to be a hypercorrection based on spelling that has now become an accepted standard. OED lists the -/h/- pronunciation second[(rld, rhld): these are appearing as little squares on this screen, but the IPA characters in my word processor, so I hope you all can see them], while Webster’s prioritizes the -/h/- pronunciations: \ˈthresh-ˌhōld, ˈthre-ˌshōld.

Why is kidnaped spelled with one p? and judgment with one e? what is the sound of one hand clapping?

I don’t know where you learned to spell, son. The Robert Louis Stevenson novel, for instance, is Kidnapped, and we may spell “judgment” that way in America, but the U.K. prefers “judgement” (though I’m not sure why you think it needs an [e]; there’s no vowel sound involved, and if you’re using it to make a soft [g] the [d] is superfluous).

And when I clap, it’s closer to a “K! K! K!” than anything else (which is sort of disturbing), so I suppose one hand clapping would sound like “k, k, k.”

With- is also a much-used prepositional prefix, unlike thresh-. Without, within, withdraw, withstand.