Maybe because nobody knows what existed the moment before the KABOOM. Some kind of time existed before the bang, right?
Not necessarily. One of the key assertions of Big Bang theory is that the Universe did have a beginning, as opposed to the Steady State theory which suggests that the Universe 'has always been and always will be'... the physical evidence points to a beginning (i.e. The Big Bang) rather than otherwise. This being the case, it is valid to assert that time too also had a beginning, hence asking what happened
before time existed is not a valid question. If time didn't exist, nothing could happen
before,
during or
after - since these are all terms we use to describe moments separated by time itself! The Big Bang was more than just the beginning of the expansion of space, it was also the beginning of time itself.
We have direct evidence for this too in the form of modern astronomical observations. There is an event horizon beyond which we cannot see, approx. 14 billion light years away (in all directions)... the reason we cannot see anything beyond it is
not because there is nothing to see and not because our telescopes are not powerful enough, but because there was a beginning to time itself and objects beyond this event horizon are invisible to us because the universe is not old enough for the light to have reached us from these objects!
And we can safely assume that there are objects beyond the event horizon - we know there is an event horizon (because we can see it in all directions) therefore there must be objects in
our visible universe that are not visible to each other (i.e. any two objects more than 14 billion light years apart from each other). So it's pretty reasonable to conclude that there must be objects beyond our event horizon too, for there is no valid reason why not. The only potentially valid alternative is that Planet Earth itself is bang-slap centre of the Universe, and that there isn't infact an
event horizon, but rather a real, physical horizon beyond which nothing exists.... but there is no evidence for that whatsoever... on the contrary, the evidence (observations) point firmly to the fact that the universe has no discernible centre - let alone that we are at that centre! - and that the universe looks the same in all directions. This is pretty firm evidence that we are not at 'THE' centre, hence the cosmological horizon that we see is almost certainly not a result of our location in space, but more because of the nature of time.