Why do people only button the top button on blazers?

People often only button the top button on blazers for a combination of style and comfort. This practice allows for a more relaxed, modern look while also providing ease of movement. Additionally, leaving the bottom button undone helps maintain the blazer's shape and prevents it from pulling or bunching when seated. It's a fashionable nod to traditional tailoring conventions, which often dictate that the top button is fastened for a polished appearance.