When a normal dropdown menu gets keyboard focus, it may show a focus outline, but otherwise doesn't react. Pressing enter, space or an arrow key will then expand the menu.
If simply focusing a dropdown also opened the menu, tabbing around a UI would be very chaotic, as menus would be popping up and possibly covering the control you were trying to tab to.
When a normal dropdown menu gets keyboard focus, it may show a focus outline, but otherwise doesn't react. Pressing enter, space or an arrow key will then expand the menu.
If simply focusing a dropdown also opened the menu, tabbing around a UI would be very chaotic, as menus would be popping up and possibly covering the control you were trying to tab to.