They are not artificially limited stock (and it doesn't make financial sense for Sony either), they are having a hard time manufacturing them fast enough just like Microsoft is. COVID has done a lot of damage in the manufacturing industries. NVIDIA is having a hard time producing enough RTX 3000 series GPU as the company that supplies the GPU RAM can't make enough of it fast enough. This is why NVIDIA temporarily canceled the 20 and 24GB RTX 3080 models as there just isn't enough RAM to go around.
Sony's only advantage over Microsoft is that they started final production way before Microsoft did (Maybe why some people are having hardware issues). MS only started making their consoles during the Summer as they were waiting on AMD for some of the GPU related stuff.
Also you have to wonder how many units are recycled before shipping due to defects. For something as complex as the PS5's APU there has to be ones they can't use and since they aren't offering a lower-specced version, binning components is not an option.
EDIT: It's likely that MS is manufacturing a single APU version, the one in the Series X. Since they offer a lower specced version any APU that can't run at series X specs but can run at series S specs will be binned to the S.
Sony does not see any profit a scalper makes so there is no financial reason to artificially limit supply.