Because there is simply no objective reason to do so.
In fact, it seems entirely silly to assume that the universe is so big and complex and wonderful that it could not possibly have created itself, and then to invent out of thin air something that is even more big and complex and wonderful to have done the job. I struggle to see how believers can be so stubbornly blind to that obvious error in reasoning.
There is no reason to believe either
Yet, you happily leave unanswered the very obvious corollary for me: There can only be two theories of God, either something created God, or God created itself.
But of course, you don't do that...you simply make a completely unwarranted exception in this one particular case. An exception you make for nothing else. And yet, I have to point out to you, that you can try, and try, and try and try again, and you will never bring me one single piece of evidence that there is an intelligence that existed without or before or in the absence of anything at all, that for some really bizarre reason...in an endless, formless, existence-less existence, suddenly felt the need to make a whole bunch of troublesome something that it would very shortly get angry at and try to kill off everybody in it. Can you provide an epistemological justification for that belief?
Your first premise is wrong. Before the BB theory, science held to the solid state universe, that it always was and always would be. Science, held that belief, the agency by which you believe all questions can be answered. So, if science could believe that about the universe, what evidence do you have that the same cannot be attributed to God ?
You are speculating about how God existed before the universe. Where do you get your information, about that ? The Bible begins at creation, so that can be your source, did you get that from a Christian ? They have no knowledge of that either. I think you just made it up.
The Bible does describe God as an intelligent being of pure light/energy, no matter involved apparently.
You have no evidence to bring to the table regarding the cause of the big bang, yet you probably believe it occurred.
The universe exists, doesn;t it ?
Anything that begins, will end. The universe will end. Scientifically, all the energy will cease to exist, all the stars will die, all life will die, nothing will be left but stone cold dead rocks drifting further and further away from one another in total darkness. Pretty damn cruel
You will be dead, all memory of you will be dead. Your life will have had absolutely no meaning, the same for every human that will have existed.
Seems to me that God offers a much better deal than your self created universe.