Because We Can
There are some folks who think through the potential results of their behavior and act on their conclusions. There are some folks who think through and don't act. There are some folks who can think through but don't. And there are some folks who simply can't think through.
I suspect that since the notion of "why" first occurred to humans there have been some folks who have been engaged in applying the notion to the existence of humans: Why are we here? And the more presonal: Why am I here? It takes a "reasonable" being to seek reasons. Before the "why" happened I suppose that humans (or their nearly human ancestors...perhaps the "why" is the essence of what makes us human) simply wandered about doing stuff like any animal or plant, without any good reason at all, except that they could!
Capacity is such a curious concept. Capacity, capability, ability; just because you can do something doesn't mean that you will or that you would or even that you do. and that's where the "why" comes in, isn't it? Other animals can, and either do or don't, depending on the prompting of their instincts, their drives, their survival needs. Humans may do things simply because they want to (and there's a can of worms!). Humans will or won't (will-not: to exercise one's will against the exercise of a capacity); humans may also consider whether they should or shouldn't, that is, they may think through the predictable or imaginable results of exercising a capacity and try to reach a conclusion about whether it is sensible, worthwhile, or reasonable (that word again!) to do so.
Women are fertile for (very roughly) twenty-five years or so. What with the advantages of modern civilization and all, a woman has the capacity to produce about twenty-five offspring (nine months gestation plus a reasonable (?) three months recovery period). I'm sure you can figure out plenty of reasons why one wouldn't or shouldn't, but not many about why one couldn't. Even living in poverty wouldn't necessarily prevent one from doing so; if nothing else, you could drop them, one after another, on the welfare system, and they couldn't stop you even if they wanted to, given that there's no law agin' it.
What about the capacity to think things through? And what about the will to act on one's conclusions?
The nature of capacity has been a subject I've contemplated for most of my life. It all began in fourth grade or so, when my parents were informed by the school system (they subsequently informed me) that I was not "working up" to my "capacity," that is, I sucked at school for reasons that had nothing to do with my intellect. But what about my emotions? Is a child nothing more than an intellectual sponge aching to be saturated with the bounty of our educational system? Or is a child also involved in a family dynamic which may either encourage or discourage that saturation? Whatever the case I soon started thinking about what it meant that I wasn't, and why that might be so. My conclusion has been, and still is, that a child is more than an intellectual sponge, and that one's mental capacities may be so absorbed in dealing with one's emotional conditions that it hasn't much room for absorbing the sparkling juices being squeezed from the fruits of knowledge by America's teachers.
And what about my fellow citizens who appear incapable of thinking things through?
Certainly there are those who simply haven't the intellectual capacity to hold all that in their minds. They are working hard enough simply getting through the day, and haven't any thing left over for contemplation. They are, basically, animals, responding to their personal survival needs in an incredibly complex environment full of agencies and paperwork and people who must be placated if they are to receive the necessities of food, clothing and shelter, and some means to occupy all the time left over when the welfare office is closed.
And the people who can think but won't, or who do think but don't act? Aren't they simply "not working up to their capacity?" Are they not simply people whose intellectual capacity is being absorbed by their emotional lives, the need to feel special, to have status, to look good, to fill the emotional voids left in them by the inevitable cruelties of their upbringing?
That leaves the precious few who can think and do act. Too few, I'm afraid, because had there been more of them, there would be less of us now. Not because the world is incapable of supporting us all - just look around, it's doing a good enough job - but because it can't go on supporting our thoughtless expansion forever. Perhaps forever is not the most important thing, but the end of the expansion will come eventually, and its end may be gentle or terrible, depending on how we, having thought things through, do act.