I think we have Jimmy Carter to “thank” for that. He said it was immoral to assassinate a leader. Somehow it’s more moral to get rid of evil dictators by going to war and killings tens of thousands.
Agreed that Carter is a joke—assassination is of course perfectly moral—the problem is a practical one—no leaders like to see it as the first choice of an adversary in time of war or even minor conflict.
It creates a “world order” where everybody is taking potshots at everybody else’s leaders.
As just one example this is how World War I got started.
Just think of the tens of thousands of lives that would have been saved and all those refugees who stayed home instead of clawing their way into Western countries.