Niche collecting? That's all I can think. Something special about them like a certain marking.
Either that, or they may be NOS receivers that were never assembled. Those bring a very hefty premium price.
There are folks around who have boxes full of NOS parts, everything but the receivers. They watch for the receivers, so they can build virtually undetectable complete rifles.
I remember a couple of lots of such receivers coming up for auction, usually a hundred at a time. They never did bring premium prices back then, because new or refurbished rifles were still available from the surplus arms dealers, which were all over the country, in those days, at very reasonable prices.
That was 50 years ago. Every once in a while, that stuff shows up.
The last bunch I saw came out of the storage sheds of a collector/seller in Nevada.
He had thousands of NOS parts for Garands, M14s, Springfield O3 variants, Thompsons, Reisings, etc.
That was 12 years ago, and one of the well know, but now deceased dealers, Terry Oliver, from the Chilliwack area, went down there to arrange "legal" sale of all of it. Most of it went to the well known parts houses in the US, because there was so much of it, and only eligible dealers were allowed to bid on it. A lot of it went to CMP, after they raised enough funding to place winning bids.
Some of the parts, such as Springfield 03, Garand bolts and other stuff, all legal with the right documentation, he brought back to Canada, something he regretted, because he could have easily moved all of it in a few weeks in the US, without the documentation hassles. It took him a couple of years to sell it in Canada.