Because He can. It would be a waste of all that ability and talent if he didn’t, don’t you think? If God is fully actualized with no potentially in him for change, then by his nature He is that which actualizes everything else. Nothing else would exist without him as the unactualized actualizer. And thus at least part of the answer is it is his nature to create everything else. Just like it is in his nature to be perfect love. Or his nature to be omnipresent. To analyze God with human descriptions is inadequate. Desire, wants, needs, or even love are inadequate terms to describe God. This is why Aquinas said we can only describe God in terms of analogy. We say that God is love. But our human understanding of love is not an adequate description of God’s love. For his ways are above our ways as Scripture says as high as heaven is above the earth.
Just because God doesn’t need anything like humans do doesn’t mean he isn’t something that acts according to his own divine nature. Do not humans act according to their nature? Do you think God should act according to some ‘silly’ little human nature like need? Or should the God not act according to a higher purpose? A divine purpose?
If God is fully actualized and has that ability to bring into existence everything else then it would have been a trajedy if he had not done so, don’t you think? That is if you think existence is a good thing. He keeps everything in existence at each moment. If He chose to he could allow everything to go out of existence. But He is not the author of a trajedy, rather quite the opposite. He created everything and saw that it was good.
Many see the highest purpose is love. St Paul himself writes that the greatest gift of all is love. If the highest purpose is love and since God is described as being love then couldn’t we ascribe that to the purpose of creation? However, just what are we talking about when we talk about love? As I said before we have to understand that our understanding of love is not the same as God’s love. It is only an anology to God since He is so above us. And His love is unfathomable to us. Yet we do see a glimpse of it through the expression of His Son who died for us.