Because a beginning implies something from nothing, and that is impossible.
You keep repeating the same errors.
The Big Bang theory never proposed “something from nothing”.
You are not even thinking logically.
What the BB does say, that the universe was infinitely hot and “dense”.
Nothingness wouldn’t have density. And if there are density, then there should also be mass and volume.
So clearly, there was “something” there at the very beginning, at the Planck Epoch.
The question should be, what is that “something”?
No one yet, except that it is extremely hot and dense, and most likely plasma.
What is this plasma made of? No ones know.
Perhaps it’s EVERYTHING - BIG & LITTLE - in the universe that’s here, today, ALL mashed up in dense and hot primordial soup? Again, no ones really know.
I am fine with not knowing what it is, at this stage.
Now while the Cyclical Universe model & Multiverse model may be theoretical possible, neither of them are probable at this present time, because no one have yet been to test either one of them.
And since they are currently untestable, both are not scientific theories, therefore both are not science.
You seemed to forget that no concept are true, until they have been rigorously tested (as required in Scientific Method), the evidence & data don’t match or support the predictions.