Have you ever noticed that governments are organized according to whatever principles were in fashion at the time they were founded? Why do the USA, France, and Mexico have Republics, while Germany and Italy have democracies, and UK has a "constitutional monarchy" and others have different forms? It's just what was in fashion at the time. The US ratified its constitution in 1787, the French shortly thereafter, and the Mexicans won independence from Spain not long after that. Those three came about at a time when the Republic, with its president and separation of powers was all the rage. To be sure, those three republics are very different in many ways, and culturally the USA is probably more different than the other two, but governmentally they represent the pinnacle of state evolution at the time they were founded. Then, later, Germany gets unified under Bismarck--who is, according to this forum, about half Freedom Fighter and half Horrible Person--and Italy gets unified, etc., then you get another form.
I think my brain is going to explode from all the inaccuracies and oversimplifications in this paragraph. First, the French constitution now in use dates back to 1958, and it was implemented because the parliamentary system, which you inaccurately call "democracy", they had been using since the 1870's was not working. The British unwritten constitution developed gradually, and stems from the fact that absolutism never took hold in Britain, whereas it did on the Continent. The U.S. Constitution is basically the British government of the time, except with a non-hereditary king and House of Lords. The German constitution dates to 1949, and has a powerless executive because the powerful President they had during the Weimar Republic was problematic, and of course the reason that had a powerful president is because that government was basically the
Kaiserreich they had had since 1871, except with an elected
kaiser, and a somewhat more powerful
reichstag. The 1949 German constitution contains many features that are direct reactions to the problems of Weimar, such as the "constructive vote of no confidence", a reaction to instability which twice led to two elections in the same year during Weimar. The Mexican constitution dates back only to 1917. The forms of government in use today are basically a product of each country's history, with revisions to existing constitutions and new constitutions a result of each individual country's problems, as evidenced by the fact that France and West Germany went in opposite directions within ten years of each other.
Seriously angus, where do you get this stuff?