>What in the WORLD is preventing these systems from getting at least 10gbps interfaces?

Price and price. Like another commenter said, there is at least one 10Gbe mini NAS out there, but it's several times more expensive.

What's the use case for the 10GbE? Is ~200MB/sec not enough?

I think the segment for these units is low price, small size, shared connectivity. The kind of thing you tuck away in your house invisibly and silently, or throw in a bag to travel with if you have a few laptops that need shared storage. People with high performance needs probably already have fast nvme local storage is probably the thinking.