There is an old story about a London taxi-driver who recognized the poet T.S. Eliot. "I've got an eye for a celebrity," he told him. "Only the other evening I picked up Bertrand Russell, and I said to him, 'Well, Lord Russell, what's it all about?' and do you know, he couldn't tell me." *
Russell, like most philosophers, saw general questions about human life and its meaning as being not just unanswerable but essentially meaningless. The standard view of secular thinkers is that we tend to see the world around us in terms of goals and purposes, and when we look at the natural world (and ourselves as part of that world) we are deluded into thinking that there must be an answer to the question of why (i.e. to what purpose) the world exists. Science can perhaps explain how it came to exist but does not address the purpose question (or non-question).
It may then be just a trick that our minds are playing on us that causes us to think that there is some kind of significant mystery (rather than just scientific puzzles) underlying existence.
Of course, religions and some (dubious) philosophical systems purport to provide answers, and this is clearly one of the factors that keep institutions like religion going - the anxiety of not knowing is lifted if one can bring oneself to believe.
Why though do some people seem to be concerned with such issues while others seem blithely indifferent? Could it be that those who are indifferent have seen through the trick our brains are playing on us, whereas others (like the taxi-driver) have not? Or perhaps people are indifferent to these issues not through insight but through a lack of understanding. Maybe they have a mental blind spot and so operate on an entirely pragmatic and superficial level. (If however, as Willard Van Orman Quine has suggested, the surface is all, then superficiality equates to deep insight!) Upbringing and education further complicate the picture.
For those who are concerned with these sorts of issues, I might as well give my (very unoriginal) best guess as to where the truth lies. I tend to the view that there is no ultimate explanation of the kind that some of us (including me) naturally crave.
* If the nature of our celebrities reflects the nature of our societies, we appear to be locked into something of a precipitous downward spiral!