I don't think non-artists realize how expensive this feature is. Not just expensive in terms of the cost for artists to create, but computationally expensive on the user's end.
If your lighting is constant, then you don't need special luminosity maps; the lighting can be baked into the regular color map. A typical game object has several texture maps: a regular color map, and then some additional maps for special effects, such as a normal map for "bumpiness" or a specularity map for "shininess." Lighting requires a luminosity map for brightness (in lit windows or pools of light produced by lamps, etc.) for areas that stay lit when external lighting changes.
Texture maps require texture memory, and that's a limited resource on the user's machine, so a game has a texture budget it must stay under. In a case where every object already has color, normal and specularity maps, adding a luminosity map means increasing your texture footprint by a third. That translates into 25% fewer different objects you can have active in a scene at the same time. For a feature which doesn't really add any play value, in Civilization's case.
Especially since Civ7 has to live on consoles and portable devices, they don't have a lot of wiggle room in their texture budget to be adding things like this. This is probably one of the ancillary reasons behind the Age system... limiting the amount of different kinds of assets that can be on screen at the same time.