@TuringEquivalent,
An interesting aspect of this question is that it breaks into parts: Why is there being at all? Given there is being why is it in the form of a universe of any kind - why does it have space and time and seemingly but not actually - things. And then finally why "this" particular universe?
If we define the term "mystery" to be a question the answer to which can be shown to be unattainable on principle then all of these questions are mysteries. However, that does not quite do it for there is the "just" word used for example in the phrase "It just is" and how it plays against the connotations of the word "mystery".
So there is one more question: Why is it that this "mystery" is so interesting to humans. We have creation myths in all the religions and even in science you have people preaching cosmology as if they were some kind of mesmerizing priests. Instead of starting with vectors and simple kinematics they "preach" the latest in cosmological theories -ooo! ahhh!-- to people who don't know what a simple function even is. They go for that ooo! Ahhh! and not understanding. They say it is important to let the layman know about science but that is disingenuous because if you look at the shows they are crafted to inspire not teach - hear the origins of the word "inspire" - it has the word spiritus at its base. They inspire that sense of "awe" that is related in a way to the notion of "mysterious" and in a sense antithetical to the letdown "It just is".
To this question I think there is an answer. This question is not a mystery. Part of it we don't know yet but we can guess.
For some reason that we do not know yet we seem to be conscious of our surroundings and make decisions about how to act in them. And our actions affect our survival probabilities. So we need two ontic - onotological capabilities. The first is to form objective models of the things around us and their ability to act upon us and our ability to act upon them and navigate our way through them in a way that skews our survival probabilities in our favor. This constitutes the "fitness" that they mention in evolution theory. Largely logic looms here. And rational choice.
But what motivates that choice. What makes us want to survive? Why not use that same logic to commit suicide? In fact why do we sometimes commit suicide at all? The answer is a second ontic - ontological capability namely our desire to be. Here our sexuality and its ability to reward is tied into our survival instinct. Condemned to death one is flooded with hormones that motivate the actions we might attempt in order to escape. Thoughts to escape are completely natural. People breaking into death row so they too can be executed are very rare. Why? Because we want to be.
Now that second feature of our ontological nature, the desire to be, can be driven into a state of total fulfillment of the survival instinct by a total realization at the fact of ones being. The fulfillment of the survival instinct is the fact of being. The desire to be is fulfilled by the fact of our being. But to realize that fact one must understand being in a way that is different from "just" that cause and effect stuff. Being can be experienced as ecstasy or the more mild "urge" to contemplate ones existence by turning on the Carl Sagan Mass and contemplate our existence by examining cosmology and becoming "amazed".
Its a fetish and it is paradoxical that those cosmos shows are actually a species of religious fetish. Either way, an experience of the contemplation of our existence is rewarded in our brain. That is why questions like this - ultimate creation mythology in religious or secular form - have their captivation. For the fact that the mystery of being is experienced as more that "just the fact that it is" is explainable by assuming the organism that we are needs to be made to want to be to survive better and hence the ontology of onto-religious contemplation like that popularized by Carl Sagan or the Sistine chapel as well as that "just the fact that it is" - the objective viewpoint that allows us to project being into things by finding the edges in the stability that is and manipulate them must both be present in optimal proportion.
The tension between these strains underlies a lot of our culture and even our ethics. This post can be seen as an attempt to mesmerize by contemplation of creation mythology that goes beyond the mere fact that "it just is" and releases those chemicals - whatever they are that cause the distinction between being amazed and being bored.
Why ask "Why is it that we are?" Because you can become aware in a fetishistic sense of your being - experience it more closely become amazed and then rewarded like a mouse by our brains. To emphasize - it is not to answer the question for it is in fact a mystery which is a question we can know we cannot know the answer to. So why ask? Fetish.
Now the truth is? Well....actually....the truth is... the fact of the matter so to speak.....It IS amazing. Those that don't see that it is amazing are actually missing the truth. You can take that to the bank but the bankers won't deposit it. La.