I'm not sure I understand the question correctly. Apple has some on offer, doesn't it?
If your definition of PC is "runs Windows" there's a business reason Apple silicon doesn't.
It's rumoured to be because Microsoft made a deal with Qualcomm: https://www.macrumors.com/2021/11/22/microsoft-qualcomm-arm-...
So if you want to buy (or build...) an arm PC with Windows then as of today it had to come with a Qualcomm SOC.
So I think the TL;DR answer for your question is really "because Qualcomm aren't super interested in the desktop market".
I think there are now some "mini PCs" but they are not internally expandable etc.
There are no major technical blockers to slapping a Qualcomm dev kit in a desktop case but the experience is not gonna be that great.