It is because Americans are notoriously rude and violent. They need a device that they can physically slam in the other person's face when ending an angry and stressful phone call - also known, in their language, as "a phone call".

/s but with a grain of truth I think: the physical action of closing a flip phone is satisfying, and the sharp noise made when closing it down one-handed comes across a little rude in more reserved cultures, but perfectly acceptable to the more boisterous Americans.