> that's not what marriage is/has been primarily about

Ancient societies' marriages we have records about were principally about economics and politics.

Maybe the poor were having love marriages. We don't know because most of our sources couldn't be bothered with them. But to the degree we have evidence, it's in even poor landowners preferring to marry children off to the owners of adjoining plots. Like, maybe that's a coïncidence. But probably not.

I mean, your kids also just didn't travel much farther than your neighbor's plots for the vast majority of their life.

If anything, political marriages are defined by a marriage outside your economic sphere of influence (which for ancient agricultural workers would generally be about a three day journey due to the ox problem), and to someone you don't know. These couples probably grew up together and went to social events like church together from birth.