I think it's to keep Force dynasties like the Skywalker's from arising. That would be so much power in the hands of a family that they'd easily become hereditary rulers or tyrants, or a superior social class who were above the law or above retribution. And because if a Force dynasty ever bred a bad 'un like Ben Solo, they'd protect the little horror because of familial love. That's a danger to the whole Galaxy right there.
So some people have complained about the emphasis in TLJ on how anyone anywhere can get born to power in the Force, instead of it being all about the Skywalker dynasty, but IMHO Force power must have been distributed randomly all along. How else would the Jedi keep going generation after generation, if they do not reproduce much? Why else would Qui-Gon carry a Midichlorians test kit in his pocket, if he didn't meet random kids with the potential to become Jedi every so often?