Why should a clickable action usually be a button rather than a styled div?

A native button provides keyboard activation, focus behavior, disabled semantics, and an accessible role with much less custom work. A div does not gain these behaviors simply by adding a click handler or visual styling. Use a link for navigation and a button for an action, with an accessible name that explains the purpose. Set type="button" when the control should not submit an enclosing form. Native semantics improve accessibility and predictability, although labels, focus appearance, and surrounding interaction still need careful design.