Comment by Kranar

Strictly speaking, modern cosmology does not treat the Big Bang as the beginning of all of existence, it's what happens when you take observations about large scale cosmology and run them backwards in time.

Based on the information we have available about our universe, we can't make predictions or formally model anything prior to a certain point in time, consequently it's convenient to treat this moment as the earliest point in time in which physics as we know it makes any sense. So while there may have been some kind of existence prior to the Big Bang, we have no way to make sense of it even at a conceptual level. Given that, we may as well treat this special point in time as the beginning of the universe as we understand it and can explain it using physics, as opposed to some absolute beginning of all of existence.