Illogical? Or Merely Not Certain?

It's because all of their logical deductions are based on an initial illogical assumption. The illogical assumption is this:

"The Universe exists so must have been created. However we assume that the creator of the Universe did not need a creator himself".

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I'm not certain I agree with your characterization of it as "illogical." I'll agree it's not necessarily entirely self-evident and may in fact be incorrect. But illogical?

Let me try to break this down:

1. The Universe exists
   
2. so must have been created
   
3. we assume that the creator of the Universe did not need a creator himself

Each in turn:

1. Yes, those who argue the point you make above take this as self-evident. It may be incorrect, though it's difficult to argue for the Universe's non-existence in a way that your average person would understand or agree with. That doesn't make it not possible, just difficult to comprehend.
   
2. This is probably the weakest leg of the whole argument for God. You've alluded to this. All that is may simply always have been. I think the assumption for creation stems from two things: a) the cretaion account in Genesis, and b) a layman's understanding of the "Big Bang" hypothesis. If incorrect -- that is, all that is has always been -- then "God as Creator" suffers a serious blow. However, if correct -- it was "created" -- then it follows that something pre-existent must have done the creating. It would be difficult to argue for the creation of the Universe truly ex nihilo without an agent of change behind it.
   
3. I think Lewis touched on this at one point in time ... I think he reasoned that if the creator of the universe needed a creator, then that creator is the self-existent God. What Lewis (and others) were doing is searching back and saying that if this is a created order, then something pre- and self-existent must have done the creation. Again, it would be hard to argue that "In the beginning was truly nothing at all, then into existence came one who then started creating things."

So, in full Spock mode, I see #2 as the chink in the armour of the "God/Creator" hypothesis. But if created, then creator required. Ultimately something pre-creation is required.

In Bones-mode I find the idea of a pre-existent and self-existent eternal God to be fully incomprehensible and therefore extraordinarily awesome. I'm reminded of one of my favorite hymns:

Oh Lord, my God, when I in awesome wonder
Consider all the worlds thy hands have made.

I see the stars, I hear the rolling thunder
Your power throughout the universe displayed

Then sings my soul, my Saviour God; to thee,
How great thou art, how great thou art!

Logical? Not entirely. Wonderful? To me, yes.