"How come scientists, have, for so long now, said that the universe began with the Big Bang..."

Not really a very long time as these things go...

But they say that because most of the evidence from the Hubble telescope and other recently invented technology....plus the math that describes it all...seems to point that way. And they keep getting 13.7 billion years as a date. When careful studies point that way, you go with it while you do MORE research.

Not very many feel they will ever be able to see back to 'before' the Big Bang...so folks just speculate.