Travel does not have to mean 'travel outside the country', so even if most attempts to decapitate a country are home-grown that doesn't mean that heads of state don't travel with a security entourage when they are within their own borders.
On top of that I do believe that heads of state already travel with a security entourage when traveling abroad.
It is not necessarily the other nations they're afraid of but their lunatics. And a small robot drone will be within the financial means of anybody with an axe to grind in the fairly near future.
Sometimes the goal of military action is not conquest. Well timed assassinations are hugely powerful (by foreign and domestic groups). One look at the presidential motorcade (or at the entourage of machine gun carrying thugs that accompany politicians in 3rd world countries) should make that quite clear.
You clearly don't watch enough action movies. The historical record of virtually all attempts on a US presidents life being the result single individual acting alone, have no relevance. :P
Why? If you were a country, would you like to deal with a decentralized/unofficial government that has no control over its citizens?