Why do Americans smile so much?

It's cultural, just like Russians not smiling. Someone striking a nerve being all smiley?

They're generally not fake! I'm pretty happy walking around and I smile when I make eye contact with people. However, creepy old men telling me "Smile, beautiful!" at the gas station always get the stink eye.

You clearly aren’t in New York

Why do <insert people group> do <insert thing> so much? I'm tired of these <people group> fake <thing>s.

This isn't a helpful way to frame a question. You may get better responses with more information on <people group>, and by challenging your assumption that they are "fake".

In many cultures, they have what is sometimes called the "undertaker's" resting face.

When you receive a smile in those cultures, you can be sure that it is earned, and not a false 'Have a Nice Day' smile.