The fundamental reasons are innately tied to *valuing leadership* and I think a lot of the reason for this is a general disconnect between leaders and the people they nominally serve and rule. Though, in the MENA case, a large portion is also devaluing the people they rule.
Like, we have this model that if only we kill that one evil dude, things will be better. And that's not really true, because the system and discontent and a multitude of other things that produced them is still there. Tojo was no longer in charge in August 1944. He didn't matter.
Killing leaders is rational if you intend to either fully genocide the enemy, or to occupy and set up your own political structures, like we did in Germany and Japan after WW2. If there is to be a negotiation, killing leaders is irrational.
Killing leaders also reduces (or rather changes) competencies within any organization. There's a reason that armies have *long* targeted officers. Now, you can do the same for militants. But it has rather... dark outcomes. (Selects for paranoia and usually intransigence)
Pardon my ignorance, MENA = Middle East - North Africa I believe