There already are some differences in government structures between countries.

Why are many of them similar? I think it’s a combination of these:

• countries that became newly independent or rewrote their constitution looked at other countries for inspiration
• Many (older) countries followed a somewhat similar path from being absolute monarchies to aristocracies (with nobles represented in a parliament which then became increasingly more powerful than the monarch) to the parliament becoming elected by all of the people to, in some countries, the monarchy being abolished and replaced by a president (essentially an elected monarch).

I think you may enjoy reading this: slatestarcodex.com/…/slightly-skew-systems-of-gov…