This reminds me of the popular idea of "love languages". I was always baffled by the idea that some people considered "gifts" to be the most important way to express love. But diamond rings seem to fit this paradigm perfectly.
Diamonds aren't "small gifts"; they're absurdly expensive ornaments. Their primary purpose is so American women can show off to other women how much money their husbands have. It's a bit like having a really expensive luxury car: it's a signal to others to show you're rich, not a way for someone to show affection to another.
Edit: About Asian cultures, I'll also add, it's just just lovers. Coworkers will frequently get each other small gifts, or gifts for everyone on the team. This is usually small food items, not anything at all expensive. They might do this if they travel somewhere. I had an Asian (i.e., he had just moved from SE Asia for this job) coworker once give everyone on our team some nice variety of tea, for instance.
Buying small gifts for coworkers is one thing, but buying lunch is a bit much. I wouldn't spend more than $5 on a gift for a coworker.
An American visiting from other company was completely dumbfounded: Why have you paid yourself?? He is your boss, it’s his responsibility to pay when he invites you to lunch!!
...And I was completely dumbfounded observing his reaction: have Americans no pride?
I'm baffled that you're baffled. Gift-giving is common in every culture, country, and religion in the world.