Seems ridiculous to think that you could get the same/more for hubs alone than wheels.
Surely if they were sold as wheels complete for the same you’d sell the hubs alone for (assuming the spokes to be cut and rims given/thrown away) then why not just price them as that and stick in the advert what they can be built up to.
It does seem ridiculous, but not many people are in the market for 26″ wheels compared to 27.5 or 29 etc. Also the older rims tend to be narrower than the current ones, and possibly not tubeless ready. If buyers only want the hubs then paying postage on parts you don’t need puts people off. Plus it is far easier to package and post a pair of hubs rather than complete wheels.
That said, I still currently roll on 26″ rims, as do many on here. So there is still a market for them.