Yeah this is clearly a American issue. It's certainly not true in Asian/South Asian countries, sure we lose touch with friends but whenever we meet there is an instant and unmistakable bond.
When I moved to the US, I couldn't understand why Americans were so "distant". I don't have the same friendships with them as I do with others. Sad but true.
Anecdotally, I've noticed this when I'm in America. Everyone is normally outwardly very friendly, but there's always the feeling that they don't give a shit about the encounter you're having and they're only even talking to you out of some perverse sense of politeness.
It was even more strange for me because often our conversations would be relatively honest and about somewhat personal topics (not just the weather, what cities we'd been to etc) which, in my experience, you only broach when you really want to discuss such things and feel a connection with a person, but, apart from a few people there was still always that feeling that people were just being polite and basically waiting for you to excuse them. YMMV.
As an American living in France, one difference that I really notice is politeness.
In the US, people are casually friendly, but not always polite. In France, politeness manifests as respectful words and actions, consistently. It doesn't hurt that formal politeness is part of the French culture. It's 'hi, how are you?' vs 'good day, sir', because most of the time the 'how are you' is not really a question, but an expression of friendliness.
There is also a fairly clear boundary between acquaintances, friends, and 'BFFs' in France. That boundary is not always clear in the US. It's not as formal as in Germany, where there is a sort of ceremony when two people want to recognize a stronger friendship. In France people (in my social circle, anyway) just switch from formal ('vous') to familiar ('tu'). I've been told that some people have a little discussion about it before switching, but I've never seen that.
I think part of this is bad translation, at least that's the case with Polish-English translation. Word "friend" is usually translated as "przyjaciel", but it really should be translated as "znajomy" or at most "kumpel", and "przyjaciel" should be translation of "best f*ing friend".
You couldn't possible have 20 "przyjaciel"s and spend enough time with them to keep the relationship alive. Same with BFFs. You can easily have 50 "kumpel"s and same with friends.
Keeping this mistranslation in mind I think European (or at least Polish) culture isn't much different from American regarding friendship.
If everyone is your friend then no-one is. I wouldn't say curt and standoffish, but it's good to keep a certain amount of distance and formality with people you're not actually friends with so that you both know where you stand.
In the United States this is considered a "normal" amount of politeness. In some other cultures it is considered weird and fake. Some Americans go abroad and complain everyone was "rude" to them not realizing the cultural differences.
You're right. Strangers are outwardly friendly for no apparent reason.
Politeness norms. For whatever reason, in the US, there's a certain base line expectation of cheeriness. Especially in service staff like waiters or flight attendants. ("Hi, welcome to McTuckey's. My name's Megan and I'll be taking care of you tonight.")
This really bothers me. I don't care if someone is "cheery" as long as they just do their job. People justify tipping service personnel (bar tenders, waitstaff) over them getting a fair salary because if they didn't tip "how would I get good service? There's no incentive without a tip." I believe some people believe "good service" means fakely cheery while to me it is "you brought me my food." If you aren't faking a smile and elevating your voice you are being "rude."
That's how I see it, though I've also learned there is value in just chit chatting.
In my mind, those folks are classified as "acquaintances", not friends. These are folks you might be friendly to, but if you are not willing to, say, take them to the hospital because they are injured, or take them in because they are on the run and someone is after them, are they really friends?
I'm from Venezuela and I must tell you that every male guy I know has very close friends. We're more outgoing and trustworthy and we've always attributed this to the fact that we're from the caribean and as such very warm. I've made acquaintances with americans and europeans alike and the difference I have noticed is that the time it takes you to share trust with other Venezuelans (also colombians and people from caribean islands) is several orders of magnitude shorter. You can meet someone and a week later you'll be interacting like you've known each other for years. I've read this elsewhere before as well, It's not a rare opinion.