1. The authors assume that God created it, and see no reason to expand on this.
Since most of the NT documents were written by converts from Judaism, it may well be that the Jewish belief in the origins of the universe were brought along with the authors. The universe was created by God from out of the waters. Go and read Genesis 1 for more details. With this assumption so clearly in the culture, there's no need to write more about it.
2. The authors see that New Creation is more important.
Regardless of the origins of the present situation, the origins of the New Creation take precedence. The consequence of Jesus' death and resurrection are to be worked through in the lives of the believers and should take more attention and focus than meditations about the origin of cosmos.
If #1 is correct, then we are left with a universe that suits classical theism. God is the Big Three omnis, with all the mind-twisting problems that this entails (e.g., theodicy).
If #2 is correct, then we are left with an absurd universe that has always been absurd and which requires an intervention to give it meaning. It also means we should probably shut up about where the universe came from and get on with the issues of being Christian. Kierkegaard was right. The real question concerns how we can be Christian, not how we can argue about it.