you want to be around other people like yourself, which you can't do in a place where there are NO OTHER PEOPLE LIKE YOU (and your neighbors act weird around you).
While this has always been sort of intuitively obvious to me, it became blindingly explicit and concrete when I joined OkCupid, the dating website that shows a "Match %" between you and everyone else on the site (it similarly shows "Friend %", so it can technically be used to find non-romantic friends too).
I live in Chicago's suburbs, and if I search in the suburbs around me, there are a few girls that are a 90%+ match.
If I instead search within Chicago's city limits, there are dozens of them. Hot ones too. Who don't want kids, like Star Trek, camping, heavy metal, and 'Eternal Sunshine of the Spotless Mind' (unfortunately I haven't quite found one with a similar 'stash yet).
Conversely, I have a friend who was originally from my area (and thus quite like me), but now is recently-single in a small town in central Pennsylvania. I felt bad because after going on a whole bunch of dates with pretty awesome girls in Chicago, I encouraged him to re-activate his profile and said I was sure he'd have similar luck. But then I actually searched for matches for myself there, and there wasn't a single 90%, and most were like 50% or below. And ugly. And fat. "Oh, shit. Sorry. I didn't know what slim pickings you had to deal with."
Surely some of this effect is cultural, and what I happen to be interested in in a girl just tends to correlate with "girl who lives in a major city".
But I think the majority of the effect is simply numerical. The density in a city is much greater than in is in the suburbs, and in turn, much greater than it is in rural areas. So the more people there are in a given area, the greater likelihood of finding one who is a match for you, even if you're an extreme outlier weirdo. Again, seems obvious, but something about seeing the explicit examples really drives it home. Even if you live in a city, unless you're on a dating website, chances are you don't use a city-wide computer database to filter for friends, so that particular benefit of density is probably not as obvious (or even as accessible) to you.
Of course, that doesn't mean living in a rural area makes finding friends/dates hopeless for extreme outlier weirdos, but it surely *is* more difficult, and probably requires (or naturally causes) some shifts in your own cultural identity or a broadening of your selectivity.