What about families that makes public transit "out of the question"? I haven't been to SFO but it looks like it's the same issue as with IAD, which is that it's smack dab in the middle of the suburbs and has very poor transit connections to the surrounding areas (although it has an actual train line, which IAD still doesn't have!)

North America is really a unique beast in this regard, though, as nowhere else is it common to intentionally make airports so difficult to get to and from. Even in Europe, certainly for remote airports you might not expect lots of train connections, but IAD and SFO don't fit that bill.

I guess you can say that Uber's value prop is that they smooth over the failures of American urban planning in exchange for their cut of flesh.