Why should Shmi be Force sensitive. Whilst it is true that some children inherit Jedi traits from there parents, it is obvious that this is not always the case.
The Council seek out potential Jedi, why would they do this if all they had to do was wait until a Jedi spawned a child then just show up to "adopt" the child?
Seems to me that the potential to be a Jedi is as mach a randomness of nature as it is an inheritable trait. The required set of conincidences required may only appear very rarely.
Rememeber also that Jedi numbers are estimated at around 10,000 as far as I can remember, that's not very many in a big galaxy. Vary rare a Jedi is indeed.