I love how my previous comment was even flagged :-)
They're artificial smiles, I guess you're programmed to use them from an early age.
I've literally seen an American coworker, from a distance, with a normal resting face, put it on once they were at a range where they'd realize it's me, then immediately drop it after passing by (I turned around precisely to see the follow up).
My impression is that they're supposed to be a sign of success. A sort of Instagram reality before its time. People smiling are usually happy and probably successful.
Plus, of course, it relaxes the interlocutor and makes them feel appreciated, up to a point.
