Why do religious people desperately want to class Atheism as a religion?

[Kingpin]

Default atheism is not a belief, it's a lack of a belief. So it can't possibly be a worldview, or anything else. It is what it is, and nothing more.

The fact that default atheism makes no claims frustrates some people I think, so they have to build up pretend worldviews to attach to it in order to have something to attack.

I wouldn't totally agree with that.  I do think Gnostic Atheism makes a claim.  There are certainly those that identify as atheist that definitively state "there is no God/Gods."  That is a claim.  Not all atheists, but there are those that are confident enough to make that statement.  It's an indefensible position, same with Gnostic Theism, but still we both know this view exists.  

But the default Atheism as you put it does not explicitly state there is not a God/Gods, just that they do not believe there is a God, it still leaves room for the possibility of a God entity existing, just not believing it.