I guess I assumed that the call center would just send the voice packets to over IP and a US carrier would actually introduce them onto the PTSN network. Do the foreign carriers actually own the US call center numbers themselves?

Sure but then if they can do it a robocaller can do it?

It would require some entity in the US to provide a number for them, and that entity could be held accountable... I think.

in which case they can properly identify themselves as a legitimate user of that US phone number? I'm not familiar with the STIR/SHAKEN stuff but surely it is possible to delegate authority to use a number.

I use a Canadian VOIP service with an American number. I'll be a little pissed if this breaks that.