First of all, "the universe" is the totality of time and space and their contents. And you can include any "other dimensions" under that heading.
If you want to ask "why?" the answer is simple: "why not?". If the puzzle is EXISTENCE, then EVERY thing that exists belongs in the explanandum.
We can't segregate unless we are presupposing a conclusion - which is exactly what many theists do.
Putting everything in entails that the universe is NOT an object, or set of objects, existing IN TIME - which is what theism tries to take it as. In
order to do that it has to help itself to the idea of time-before-time - a series of moments when no objects existed. There is nothing wrong with that
idea itself, but there is no use in positing it.
The only reason for positing it is to have a time in which the act of creation can occur. This is needed because "creation" (and causation
generally) is a temporal concept. A creator PRECEDES and PRE-EXISTS the creation. If he didn't, there would be as much reason to suppose the
"creation" created "the creator". It is the temporal difference that is vital in distinguishing cause and effect.
But the temporal nature of causal relations dooms any attempt to make sense of a "creator of the universe". No "creator" can ever be "outside
time". But even if we help ourselves to a "time before time" (before the "created" universe), we still have the problem: WHAT "created" THIS
time? Whoever it was, needed to be IN TIME to do his "creating". This problem applies quite generally not just to the "time before time" proposal,
but to any similar way to evade the issue by appealing to "higher" or "other dimensions". Put the creator in the 47th dimension, if you want. He
still can't have created it himself.
The problem is also not solved by just saying that he existed "from eternity" - ie. making the time-before-time of infinite extent. The question
"what created THAT?" is as good as it is in the case of the universe itself.
The question that needs to be asked is: is there any genuine mystery in an uncaused universe? It is HERE that the issue of tensed & untensed theories
of time is relevant. My diagnosis of the theist's predicament is that his question "what CAUSED it all?" is premised on the assumption there must
have been nothing AND THEN the universe happened. He is assuming the time-before-time, even if he wants to avoid admitting it. But why imagine there
MUST be a t-b-t? It is because of the tensed view of time. The difference between the tensed & tenseless views (and why the tensed one is incoherent)
is explained here:
Mellor on Tense
If there was a First Event, which is now PAST, but was once PRESENT, then it must also have been FUTURE - that is the theist's intuition. But it is
simply false. The inference depends on the tensed theory, and the tensed theory is just incoherent, as it tacitly assumes the tenseless alternative to
make sense of itself. And accepting tenselessness exorcises the illusion that there is any mystery about the universe having a First Event that was
not PRECEDED by anything, and not in any CAUSAL relation to anything.
One other thing: if you think the trouble is that our words "cause" and "create" are "merely human" and "can't comprehend God", etc - then
just don't use them. You haven't evaded the argument, you just accepted it.