resizeEvent DOES get called, but not when the bars are hidden/shown.
Sorry, I misread your original post as saying the event was not called at all.

QLabel and similar widgets have a preferred size and a size policy. They won't necessarily grow or shrink if the size policy won't let them. So the layout might be just adding / removing space around them instead of resizing.

Then there is this from QWidget::sizePolicy():

If there is a QLayout that manages this widget's children, the size policy specified by that layout is used. If there is no such QLayout, the result of this function is used.