A company made one of these a while ago, and it was awful. Linus Tech Tips has a video on it. The market is just too small and there are so many better options that it’s not worthwhile to make.
The overarching point is the same. The utility of such a device is so small and has such a small potential audience that no one would make one (at least with any reasonable quality).
If you really want it, get a USB hub and full each port with an SD card reader.
I think you described the raid-0 device I was describing. Whoever came up with the idea for that device, I can say with confidence, does not understand the failure rate of SD cards and the mathematics of why raid-0 is a bad idea.
You are right though in this "overarching" point that it is very niche and you can probably have a decent solution with USB. I would think that might get a little more clumsy and consume more power than a single board exposing all slots though.
