Distances really don't have anything to do with it. If a town is too small to have public transport, it's simply too small. It's the same for the smallest European countries as it is for the US.
That can still allow for mediocre public transit. A bus that only stops every 3 hours can still be a workable commute, but not if you must travel 1-2 hours each way.
No one ends up using “every-3-hours” routes in day-to-day life. Every 3 hours can work for the tourist who doesn’t want to rent a car, but people with jobs won’t accept that if they miss the bus, the next one is in 3 hours. These routes effectively become useless and contribute to the narrative that public transit isn’t practical.