There aren't enough people so rich that they can live in country estates and remote coastal "summer" residences all year (so, neither parent has a job they need to be at) while sending their kids to rural boarding schools to show up in those stats, I expect. Plus you may have trouble pinning down where they actually live most of the time, even if you tried to account for them.
Beyond that, given rural areas have been subject to over a century of severe brain-drain and intelligence is fairly heritable, I'd expect that to overwhelm any benefits of country living, if you tried to measure the effect for a general rural population. You'd need a twin study or something like that to sort it all out, i.e. a bunch of twins where one grew up in the country, one in the city or 'burbs or whatever you're comparing it to.
Beyond that, given rural areas have been subject to over a century of severe brain-drain and intelligence is fairly heritable, I'd expect that to overwhelm any benefits of country living, if you tried to measure the effect for a general rural population. You'd need a twin study or something like that to sort it all out, i.e. a bunch of twins where one grew up in the country, one in the city or 'burbs or whatever you're comparing it to.