I'll let you in on a little secret: Heads of states are generally not assassination targets by other country states.
Why? If you were a country, would you like to deal with a decentralized/unofficial government that has no control over its citizens?
Why? If you were a country, would you like to deal with a decentralized/unofficial government that has no control over its citizens?
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.