Why don't most-all skins show up for admin use?

The only skin I see for admins is Basic. Are the other skins somehow Customer only?

When I go to skin vendors, they don’t go out of their way to say that they aren’t for admins, so I have no discouragement for Admins there.

There is only one skin for admin.

Such as by copying a file or changing a config line?

No. The only way is to create your own custom admin skin.