Steam is notoriously difficult to get mods to work with it. Most people will use the Origin or GOG versions of Spore, which will typically work with mods. Most mods also will require GA to work anyway, since it provides so many more things for modders to work with than Core and C&C.
(or so the modders say, but since I haven’t made any Spore mods, I can’t tell you that myself)