I know Notre Dame and Ohio State are scheduled to face one another in a few years but with being in neighboring states and not that far apart, why have Notre Dame and Ohio State played so few games against each other?

I think Notre Dame was focused on maintaining tradition and since they had so many tough games already with more meaning they never felt like forcing another tough game between Ohio state was necessary.
Also Notre Dame wants to be National not just regional and replacing a team from another region with another northern team wouldn't be smart business.
Michigan and Purdue were rivalry games in the north/Midwest.

I heard something about a recruiting truce between ND and Ohio state. They didn't recruit the same kids for a while for some reason.