Because we scientifically know that everything that has a beginning, has a cause.

I can try and confuse the subject by coming up with all sorts of fairy tales about universes and so on and so forth, and while 'junk science' may not be the best way to describe a theory like that, 'useless science' would definately describe it. The only reason anyone takes a theory like that seriously, is that people are convinced that there's no creator.

Outside of that context, the theory is laughable. And still does nothing to explain the ultimate cause, and proves the scientists are at least willing to admit that our universe must have had a pre-bang cause. So you helped answer your own question.

I'm sure they've come up with all sorts of convincing math to back it up, but I would love to know where exactly these other universes are....If they aren't in our universe, then...where?