Why are capacitor sizes rarely expressed in nanofarads?
They don't.  I'm American and I use nano for Farads like any other prefix, which is when doing so would put 1-3 digits to the left of the point.

A long time ago, nF wasn't used much for whatever reason.  This wasn't a American thing as much as a long time ago thing.  I've even seen pF referred to as &micro;&micro;F.  Even worse, that was sometimes abbreviated to "mF".  Over time these ancient designations have been depricated, and now we routinely use mF, &micro;F, nF, and pF.

Other places where high technology meant a metal plow behind your horse while we were using vacuum tubes didn't pass thru this phase.  When they got around to learning to spell "EE", the common metric prefixes for powers of 1000 were already in common use.  One disadvantage of being early leaders is that you can end up with some baggage when common conventions eventually emerge.

I have mentioned this exact issue to some distributors when seeing a dropdown list like you show.  They say that there are still a few obstinate old farts out there that get upset or confused when you show them nF and mF, so they avoid those.  Time will fix this, and largely already has.