IMO, the big American bias in heroic fantasy RPG including D&D is how empty (most) settings are. If you travel (nowadays by car) in rural Europe, you'd find village every 5-10km, turns out that people walking to their field don't like to spend more than 1h commuting. While on some high fantasy map, you have like 3 day of walk through a dangerous forest, or an endless plain without much settlements.
Also it's worth mentioning that many European major roads/highway have been built at first by the Roman, and have been modernized through history. So again, middle age wasn't as empty, salvage as many D&D settings. Which indeed looks more like frontier era US.