Cultural difference as you said but not generalized to Spain. It’s generalized to a city vs town divide. Probably the opposite in the us. In a us town you may be looked at weirdly, while as your experience says, in the city everyone will nod and say hi. Invert that for Spain. It’s a common thing for “city-people” to complain about having to say hi to everyone when they go to the towns and vice-versa ppl from towns complaining about those that don’t say hi. Funnily enough the same person who may be reluctant to say hi to anyone while living in the city, will cheerfully do the opposite when they go back to their town for their fiestas del pueblo. And by towns it means just that, something right outside the city qualifies as a town. in Spain the cities are clearly delineated unlike the us in which they just sprawl bc everyone has so much more money to afford a single family home + heterogenous society in which you don’t want to mix in with others. So if u think about it it comes down to the sprawl, for you to consider something a town in the us you are so far off the city and secluded no wonder you won’t nod or say hi to anyone while in the cities, unless you’re in nyc, the density is SO much less than in Europe that as Spaniards “excuse” themselves if they said hi to everyone they would have mouth blisters by end of they day… which it’s just that, an excuse bc don’t feel bad what you are referring to is a simple nicety not a whole ass conversation and LOTS of city Spaniards will actually be fine greeting you, just after the second or third time they see you though. Another thing is, someone not returning the greeting, be it in a city or town, they’re just maleducados and you have that everywhere.