It may be that God created much of the universe to make a special location (Earth) able to sustain life. I'm speaking as an old-earth creationist who on the one hand believes God had a direct hand in creating all things (including breathing life into man -- I'm not a theistic evolutionist who believes God created a system of natural selection and sat back and let nature do all the rest). But at the same time, I also believe God could have put a lot into the design of the Earth so that the Earth itself can have a universe custom made for it to "inhabit" just like the Earth is custom made for life to inhabit.
Here are some possible reasons:
1. God just likes to create stuff. Asking why God created the universe is kind of like asking why God made flowers pretty. Maybe He's just into creating cool stuff.
2. There is a theory that God likes to re-use things He already made. My take on why organisms often seem a lot alike is not because of natural selection, but because they were made by the same Designer. Can't God do the same thing with other stuff? It may be that the cosmic radiation from other galaxies provided the substance -- at just the right mass and energy and timeline -- for our galaxy to form. For our sun to form. For the planets around our sun to form.
3. There are all kinds of things to show that our milky way galaxy was specially made so that one planet within it can support life. For example, most stars in the milky way galaxy have orbits that are highly elliptical. Just by observing our nearest most stars we can see how special our star's circular orbit is (much less volatility wreaked onto the planets that orbit it).
Those are interesting ideas, and you are very right that ultimately the mind of God is unknowable.
My main issue is that, as you point out, everything seems to have been very well designed for us. The laws of physics seem perfectly written to support beings like us, and they also have a beautiful simplicity to them.
The universe beyond our solar system doesn’t seem designed with us in mind. Perhaps it isn’t. I suppose my worry is how much we matter to a God who has also created trillions of other worlds, if those worlds are not also eventually meant for us.