Why do some thinkers claim that they know about what is?

Stephen Hawking has apparently claimed that there is no god and no creation; and that we have only one life each. I had previously read his claim that there is no need for God. A great speculative thinker, working within the currently popular mechanistic material paradigm, knows all about the Cosmos, how it came about, and of Man’s place in it! And his peers speculate about multiverses and other un-established matters without proof based upon the scientific method – the very method required to be satisfied by speculative thinkers operating beyond mechanistic relationships, and the ‘things’ considered to be the matter involved in these relationships.

What are the issues implicated in the above stance?
• Do Hawking et al claim to be infallible? Not so. They amend their speculations from time to time (but within their safe-houses).
• Might they be fearful of what lies beyond their limited paradigm, knowing that the scientific method – as we know it – cannot deal with the ephemeral, the un-repeatable, the unverifiable? Probably!
• How does one prove the negative? That there is no god, no creation, and that human Earthly existence is not different from the lives of insects? Impossible!
• Can anyone prove the existence of a creator god and the rest of that paradigm? Certainly not through any accepted methodology.
• Is postulating an un-provable and undeniable speculative ‘explanation’ about an assumed mechanistic material domain against another un-provable and undeniable speculative belief about a part-ethereal domain (with or without a creator god) likely to further our knowledge about our sojourn on this rock called Earth?

What about searching, seeking, without claiming to know it all?