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.
Unless friendliness is an expected social signal, and there's a separate signal, known to the local culture, but not to you, that says, "I'm being nice but we aren't actually friends".
Same happened when I had interviews for American companies. They were so excited about everything and overly enthusiastic I can't help but thing I'm being interviewed by psychos.
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.
I bet they'd recognize you if you were perceived as someone potentially "useful" to them. I noticed the friendships here are often centered around very pragmatic sense of utility.
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."
I was mainly referring to the convenience/activity/mentor friends described in the article.