Not being able to find someone doesn't mean they're dead. I really don't know how to make that simpler for you. You also might want to read the Wiki page on
death in absentia. A quote:
We don't know where bin laden was, we don't know where he is supposed to have died, we don't know how he's supposed to have died and I don't see how it is more probable that bin Laden is dead than alive.
Ok,
It basically boils down to:
How the individual is thought to have died (murder, suicide, accident, etc.)
the balance of probabilities that make it more likely than not that the individual is dead
I have one, a way how he is supposed to have died: war, ... being the prime target of carpet bombing, special forces and thousands of soldiers trying to find you.
The balance of probabilities is either:
1. he is dead,
or
2. he flees from hundreds of bombs aimed for him, while getting dialysis in the desert, avoiding special forces, having trained followers that can resist torture for years without giving any hint of where he is or whether he is alive and staying out of sight of thousands of troops, satelites and highly equiped national security agencies in several different countries, while living the life of a nomad from foedal ages.
I would go for 1 as most probable.
If war doesn't sound like a reasonable explanation for death, then I don't know what to tell you.
As far as not knowing where Bin Laden was, if that is true, I have a different question: why are we in Afghanistan and Irak? I thought getting Bin Laden was the whole point of going to Afghanistan. So I presume they do have an idea where he was.