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."
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.")