Maybe because they know otherwise? Nah, couldn't be that.
And being serious, if UBL was killed (via air-strike, bullet or through disease brought on by being force to eek out his survival in the Stan/Pak border regions)....Sr. AQ elements would be more then aware of such.
AQ does not hide their leaders deaths. They celebrate them. It is how they honor them. It would be seen as a great insult to UBL had he passed and AQ failed to recognize his passing....Again, it is much easier to spin the truth....then to hide a lie....
And understand it is not in your enemies interest to show you what you want to know....AQ/UBL would be happy to think that some of us wonder at times if we are chasing a ghost..
We'll put guns or a JDAM on UBL worthless as$ one of these days...just as we did Saddam & Zarqawi and another dozen HVTs we've chased for 5 years... (and there were many who insisted they were either dead or didn't exist altogether as well).
I think that logic has a couple of holes.
First, I do not agree that other Al Qaeda leaders would know if he had died in an air strike. They might eventually have figured it out, but they would not necessarily know when it happened. If he was in a remote region, he might not have been with Zawahiri for instance. It could have taken some time for them to realize he was even missing.
Second, the benefit of martyrdom might very well be outweighed if Bin Laden had died at the hands of the Great Satan...