It's a good question, but appeal to a god doesn't get you out of it. Why is there a god rather than no god? Why does it 'just have to' exist just cuz? Why can't a changing universe be the 'necessary existing' thing just cuz?
SOMETHING can exist "just cuz" . . . . in my way of thinking, postulating God makes it easier to transition from necessarily existing to particular things that exist .
Suppose there wasn't anything, nothing existed at all. That's a logical possibility, it doesn't have a self contradiction in it, anyway.
Would the concept of numbers still be valid? One, two, three . . . Infinity?
Lets suppose it would be, if only there was something to interpret them.
Numbers can be interpreted to have meaning. "Mary had a little lamb" is really only a set of numbers in your computer. Clearly, however, there are alternate interpretation schemes using numbers, an infinite number of different ways to encode the phrase "Mary had a little lamb" is possible to be devised.
Alternate interpretation schemes themselves can be described in numbers. Every program your computer ever runs are encoded in numbers.
So there are interpretation schemes, embedded in numbers, that can read numbers.
In order to express much, and comprehend much, truly large numbers become involved, but hey, all the numbers needed for that necessarily exist.
In order for the idea of God to become reality . . . that is, for there to be an entity of infinite extent that holds the entire universe as a great thought in His mind . . . all that has to happen is for the numbers that represent the ability to do that to be represented as doing that with the numbers that represent the description of all things that exist.
And for infinities to be as real as ordinary numbers, of course.
This to me is about the only way I can conceive of for reality to exist, and after all, reality does exist.
So that is what I mean when I say it is easier for me to imagine reality existing because God exists than to imagine reality existing without God.