IMO, just because theres a god for something, that does not mean that mortals cannot discover on their own things related to that something, given the right conditions and sufficient knowledge.

Now, I get that demi-gods created for that specific thing would find themselves useless if said thing could be done without them, but we gotta remember that just because someone else tresspassed on our Domain, it doesn't mean that he is better that us at that thing.

Maybe mortals really did discover the wheel or something resembling it, but couldn't really use it effectively because it wasn't influenced(?) by the power of said demigod/god in charge of it in the first place.