You're right, and I think these sandbox games should have the VN tag.
I think there is a confusion in our definition of what is a VN so here is how I see it.
Visual Novel and Sandbox are not mutually exclusive, there are not even in the same category.
"Visual Novel" is a storytelling format (text accompanied by something visual like images), "Sandbox" is a game mechanic.
There is nothing that force you to choose between both because they doesn't affect your game the same way, an exemple:
- I could replace the VN part with interactive movies, the sandbox part would not be affected.
- I could replace the sandbox part with a puzzle gameplay there would still have a VN part between each puzzle.
I think what is wrong here is that on this forum the VN tag is placed above the others visually speaking. I'm okay to replace the VN tag with "Hybrid" or something like this, but I don't think it exist.