[hvusslax]

It is simply an historical artifact and by no means a constitutional necessity.

Sweden is an interesting case, a constitutional monarchy in theory but where the monarch has been explicitly stripped of all powers, whereas many other European monarchies maintain a legal fiction of constitutional relevance for their royal families. Sweden could abolish the monarchy completely today and it would not affect anything about how the country runs. They also have a unicameral parliament.

Switzerland also doesn't have a traditional head of state, that role is held collectively by a seven-member council.